diff --git a/CHANGELOG.md b/CHANGELOG.md
index d14c5b5e..89ce254b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,7 +1,7 @@
 # Release Notes for Feed Me
 
 ## Unreleased
-
+- Added support for importing into relational fields that have custom sources selected. ([#1504](https://github.com/craftcms/feed-me/pull/1504))
 - Fixed a bug that could occur when uploading files to an Assets field from an external URL and a new filename is provided, but we can't determine the remote file's extension. ([#1506](https://github.com/craftcms/feed-me/pull/1506))
 
 ## 6.3.0 - 2024-08-14
diff --git a/src/fields/Categories.php b/src/fields/Categories.php
index 387b2fdf..dfe235f0 100644
--- a/src/fields/Categories.php
+++ b/src/fields/Categories.php
@@ -6,13 +6,16 @@
 use Craft;
 use craft\base\Element as BaseElement;
 use craft\elements\Category as CategoryElement;
+use craft\elements\conditions\ElementConditionInterface;
 use craft\feedme\base\Field;
 use craft\feedme\base\FieldInterface;
 use craft\feedme\helpers\DataHelper;
 use craft\feedme\Plugin;
 use craft\fields\Categories as CategoriesField;
 use craft\helpers\Db;
+use craft\helpers\ElementHelper;
 use craft\helpers\Json;
+use craft\services\ElementSources;
 
 /**
  *
@@ -90,9 +93,19 @@ public function parseField(): mixed
         $node = Hash::get($this->fieldInfo, 'node');
         $nodeKey = null;
 
+        $groupId = null;
+        $customSource = null;
         // Get source id's for connecting
-        [, $groupUid] = explode(':', $source);
-        $groupId = Db::idByUid('{{%categorygroups}}', $groupUid);
+        if (str_starts_with($source, 'custom:')) {
+            $customSource = ElementHelper::findSource(CategoryElement::class, $source, ElementSources::CONTEXT_FIELD);
+            // make sure $create is nullified; we don't want to create categories for custom sources
+            // because of ensuring all the conditions are met
+            // for example, if there's condition level == 2, then how do we ensure that and (more importantly) how do we choose a parent
+            $create = null;
+        } else {
+            [, $groupUid] = explode(':', $source);
+            $groupId = Db::idByUid('{{%categorygroups}}', $groupUid);
+        }
 
         $foundElements = [];
 
@@ -131,13 +144,29 @@ public function parseField(): mixed
             }
 
             $criteria['status'] = null;
-            $criteria['groupId'] = $groupId;
             $criteria['limit'] = $limit;
             $criteria[$match] = $dataValue;
 
             Craft::configure($query, $criteria);
 
-            Plugin::info('Search for existing category with query `{i}`', ['i' => Json::encode($criteria)]);
+            if (!empty($customSource)) {
+                $conditionsService = Craft::$app->getConditions();
+                /** @var ElementConditionInterface $sourceCondition */
+                $sourceCondition = $conditionsService->createCondition($customSource['condition']);
+                $sourceCondition->modifyQuery($query);
+            }
+
+            // we're getting the criteria from conditions now too, so they are not included in the $criteria array;
+            // so, we get all the query criteria, filter out any empty or boolean ones and only show the ones that look to be filled out
+            $showCriteria = $criteria;
+            $allCriteria = $query->getCriteria();
+            foreach ($allCriteria as $key => $criterion) {
+                if (!empty($criterion) && !is_bool($criterion)) {
+                    $showCriteria[$key] = $criterion;
+                }
+            }
+
+            Plugin::info('Search for existing category with query `{i}`', ['i' => Json::encode($showCriteria)]);
 
             $ids = $query->ids();
 
diff --git a/src/fields/Entries.php b/src/fields/Entries.php
index b903fa0c..c6386467 100644
--- a/src/fields/Entries.php
+++ b/src/fields/Entries.php
@@ -5,6 +5,7 @@
 use Cake\Utility\Hash;
 use Craft;
 use craft\base\Element as BaseElement;
+use craft\elements\conditions\ElementConditionInterface;
 use craft\elements\Entry as EntryElement;
 use craft\errors\ElementNotFoundException;
 use craft\feedme\base\Field;
@@ -13,7 +14,9 @@
 use craft\feedme\Plugin;
 use craft\fields\Entries as EntriesField;
 use craft\helpers\Db;
+use craft\helpers\ElementHelper;
 use craft\helpers\Json;
+use craft\services\ElementSources;
 use Throwable;
 use yii\base\Exception;
 
@@ -94,6 +97,7 @@ public function parseField(): mixed
         $nodeKey = null;
 
         $sectionIds = [];
+        $customSources = [];
 
         if (is_array($sources)) {
             foreach ($sources as $source) {
@@ -103,10 +107,21 @@ public function parseField(): mixed
                         $sectionIds[] = ($section->type == 'single') ? $section->id : '';
                     }
                 } else {
-                    [, $uid] = explode(':', $source);
-                    $sectionIds[] = Db::idByUid('{{%sections}}', $uid);
+                    // if the source starts with "custom:", it's a custom source, and we can't treat it like a section
+                    if (str_starts_with($source, 'custom:')) {
+                        $customSources[] = ElementHelper::findSource(EntryElement::class, $source, ElementSources::CONTEXT_FIELD);
+                    } else {
+                        [, $uid] = explode(':', $source);
+                        $sectionIds[] = Db::idByUid('{{%sections}}', $uid);
+                    }
                 }
             }
+
+            // if there's only one source, and it's a custom source, make sure $create is nullified;
+            // we don't want to create entries for custom sources because of ensuring all the conditions are met
+            if (count($sources) == 1 && !empty($customSources)) {
+                $create = null;
+            }
         } elseif ($sources === '*') {
             $sectionIds = null;
         }
@@ -148,13 +163,38 @@ public function parseField(): mixed
             }
 
             $criteria['status'] = null;
-            $criteria['sectionId'] = $sectionIds;
             $criteria['limit'] = $limit;
             $criteria[$match] = $dataValue;
 
             Craft::configure($query, $criteria);
 
-            Plugin::info('Search for existing entry with query `{i}`', ['i' => Json::encode($criteria)]);
+            // if we have any custom sources, we want to modify the query to account for those
+            if (!empty($customSources)) {
+                $conditionsService = Craft::$app->getConditions();
+                foreach ($customSources as $customSource) {
+                    /** @var ElementConditionInterface $sourceCondition */
+                    $sourceCondition = $conditionsService->createCondition($customSource['condition']);
+                    $sourceCondition->modifyQuery($query);
+                }
+            }
+
+            if (!empty($sectionIds)) {
+                // now that the custom sources have been accounted for,
+                // we can adjust the section id to include any regular, section sources (section ids)
+                $query->sectionId = array_merge($query->sectionId ?? [], $sectionIds);
+            }
+
+            // we're getting the criteria from conditions now too, so they are not included in the $criteria array;
+            // so, we get all the query criteria, filter out any empty or boolean ones and only show the ones that look to be filled out
+            $showCriteria = $criteria;
+            $allCriteria = $query->getCriteria();
+            foreach ($allCriteria as $key => $criterion) {
+                if (!empty($criterion) && !is_bool($criterion)) {
+                    $showCriteria[$key] = $criterion;
+                }
+            }
+
+            Plugin::info('Search for existing entry with query `{i}`', ['i' => Json::encode($showCriteria)]);
 
             $ids = $query->ids();
 
diff --git a/src/fields/Users.php b/src/fields/Users.php
index d00d189c..86782834 100644
--- a/src/fields/Users.php
+++ b/src/fields/Users.php
@@ -5,6 +5,7 @@
 use Cake\Utility\Hash;
 use Craft;
 use craft\base\Element as BaseElement;
+use craft\elements\conditions\ElementConditionInterface;
 use craft\elements\db\UserQuery;
 use craft\elements\User as UserElement;
 use craft\errors\ElementNotFoundException;
@@ -14,7 +15,9 @@
 use craft\feedme\Plugin;
 use craft\fields\Users as UsersField;
 use craft\helpers\Db;
+use craft\helpers\ElementHelper;
 use craft\helpers\Json;
+use craft\services\ElementSources;
 use Throwable;
 use yii\base\Exception;
 
@@ -86,12 +89,16 @@ public function parseField(): mixed
 
         // Get source id's for connecting
         $groupIds = [];
+        $customSources = [];
         $isAdmin = false;
         $status = null;
 
         if (is_array($sources)) {
             // go through sources that start with "group:" and get group uid for those
             foreach ($sources as $source) {
+                if (str_starts_with($source, 'custom:')) {
+                    $customSources[] = ElementHelper::findSource(UserElement::class, $source, ElementSources::CONTEXT_MODAL);
+                }
                 if (str_starts_with($source, 'group:')) {
                     [, $uid] = explode(':', $source);
                     $groupIds[] = Db::idByUid('{{%usergroups}}', $uid);
@@ -111,6 +118,12 @@ public function parseField(): mixed
             if (in_array(UserElement::STATUS_INACTIVE, $sources, true)) {
                 $status[] = UserElement::STATUS_INACTIVE;
             }
+
+            // if there's only one source, and it's a custom source, make sure $create is nullified;
+            // we don't want to create users for custom sources because of ensuring all the conditions are met
+            if (count($sources) == 1 && !empty($customSources)) {
+                $create = null;
+            }
         } elseif ($sources === '*') {
             $groupIds = null;
         }
@@ -138,13 +151,12 @@ public function parseField(): mixed
 
             $ids = [];
             $criteria['status'] = null;
-            $criteria['groupId'] = $groupIds;
             $criteria['limit'] = $limit;
             $criteria[$match] = $dataValue;
 
             // If the only source for the Users field is "admins" we don't have to bother with this query.
-            if (!($isAdmin && empty($groupIds))) {
-                $ids = $this->_findUsers($criteria);
+            if (!($isAdmin && empty($groupIds) && empty($customSources))) {
+                $ids = $this->_findUsers($criteria, $groupIds, $customSources);
                 $foundElements = array_merge($foundElements, $ids);
             }
 
@@ -153,7 +165,7 @@ public function parseField(): mixed
             // So if we haven't found a match with the previous query, and field sources contains "admins",
             // we have to look for the user among admins too.
             if ($isAdmin && count($ids) === 0) {
-                unset($criteria['groupId']);
+                $criteria['groupId'] = null;
                 $criteria['admin'] = true;
 
                 $ids = $this->_findUsers($criteria);
@@ -241,12 +253,38 @@ private function _createElement($dataValue, $groupId): ?int
      * @param $criteria
      * @return array|int[]
      */
-    private function _findUsers($criteria): array
+    private function _findUsers($criteria, $groupIds = null, $customSources = null): array
     {
         $query = UserElement::find();
         Craft::configure($query, $criteria);
 
-        Plugin::info('Search for existing user with query `{i}`', ['i' => json_encode($criteria)]);
+        // if we have any custom sources, we want to modify the query to account for those
+        if (!empty($customSources)) {
+            $conditionsService = Craft::$app->getConditions();
+            foreach ($customSources as $customSource) {
+                /** @var ElementConditionInterface $sourceCondition */
+                $sourceCondition = $conditionsService->createCondition($customSource['condition']);
+                $sourceCondition->modifyQuery($query);
+            }
+        }
+
+        if (!empty($groupIds)) {
+            // now that the custom sources have been accounted for,
+            // we can adjust the group id to include any regular, group sources (group ids)
+            $query->groupId = array_merge($query->groupId ?? [], $groupIds);
+        }
+
+        // we're getting the criteria from conditions now too, so they are not included in the $criteria array;
+        // so, we get all the query criteria, filter out any empty or boolean ones and only show the ones that look to be filled out
+        $showCriteria = $criteria;
+        $allCriteria = $query->getCriteria();
+        foreach ($allCriteria as $key => $criterion) {
+            if (!empty($criterion) && !is_bool($criterion)) {
+                $showCriteria[$key] = $criterion;
+            }
+        }
+
+        Plugin::info('Search for existing user with query `{i}`', ['i' => json_encode($showCriteria)]);
 
         $ids = $query->ids();
 
diff --git a/src/templates/_includes/fields/categories.html b/src/templates/_includes/fields/categories.html
index e1f03c6d..33bd15fc 100644
--- a/src/templates/_includes/fields/categories.html
+++ b/src/templates/_includes/fields/categories.html
@@ -60,12 +60,16 @@
         }) }}
     </div>
 
-    <div class="element-create">
-        {{ feedMeMacro.checkbox({
-            label: 'Create categories if they do not exist'|t('feed-me'),
-            name: 'options[create]',
-            value: 1,
-            checked: hash_get(feed.fieldMapping, optionsPath ~ '.create') ?: '',
-        }) }}
-    </div>
+    {# don't allow crating new categories if the source is custom, because we can't ensure all the conditions will be met;
+    e.g. level, date created or updated etc #}
+    {% if field is defined and field is not empty and field.source starts with 'custom:' == false %}
+        <div class="element-create">
+            {{ feedMeMacro.checkbox({
+                label: 'Create categories if they do not exist'|t('feed-me'),
+                name: 'options[create]',
+                value: 1,
+                checked: hash_get(feed.fieldMapping, optionsPath ~ '.create') ?: '',
+            }) }}
+        </div>
+    {% endif %}
 {% endblock %}
diff --git a/src/templates/_includes/fields/entries.html b/src/templates/_includes/fields/entries.html
index c9ce05a8..9b83b210 100644
--- a/src/templates/_includes/fields/entries.html
+++ b/src/templates/_includes/fields/entries.html
@@ -62,14 +62,17 @@
         }) }}
     </div>
 
-    <div class="element-create">
-        {{ feedMeMacro.checkbox({
-            label: 'Create entries if they do not exist'|t('feed-me'),
-            name: 'options[create]',
-            value: 1,
-            checked: hash_get(feed.fieldMapping, optionsPath ~ '.create') ?: '',
-        }) }}
-    </div>
+    {# don't allow crating new entries if the only selected source is custom, because we can't ensure all the conditions will be met #}
+    {% if field is defined and craft.feedme.fieldHasOnlyCustomSources(field) == false %}
+        <div class="element-create">
+            {{ feedMeMacro.checkbox({
+                label: 'Create entries if they do not exist'|t('feed-me'),
+                name: 'options[create]',
+                value: 1,
+                checked: hash_get(feed.fieldMapping, optionsPath ~ '.create') ?: '',
+            }) }}
+        </div>
+    {% endif %}
 
     {% if field %}
         <div class="element-groups">
diff --git a/src/templates/_includes/fields/users.html b/src/templates/_includes/fields/users.html
index 6115f6e5..cff09a80 100644
--- a/src/templates/_includes/fields/users.html
+++ b/src/templates/_includes/fields/users.html
@@ -61,12 +61,15 @@
         }) }}
     </div>
 
-    <div class="element-create">
-        {{ feedMeMacro.checkbox({
-            label: 'Create users if they do not exist'|t('feed-me'),
-            name: 'options[create]',
-            value: 1,
-            checked: hash_get(feed.fieldMapping, optionsPath ~ '.create') ?: '',
-        }) }}
-    </div>
+    {# don't allow crating new entries if the only selected source is custom, because we can't ensure all the conditions will be met #}
+    {% if field is defined and craft.feedme.fieldHasOnlyCustomSources(field) == false %}
+        <div class="element-create">
+            {{ feedMeMacro.checkbox({
+                label: 'Create users if they do not exist'|t('feed-me'),
+                name: 'options[create]',
+                value: 1,
+                checked: hash_get(feed.fieldMapping, optionsPath ~ '.create') ?: '',
+            }) }}
+        </div>
+    {% endif %}
 {% endblock %}
diff --git a/src/web/twig/variables/FeedMeVariable.php b/src/web/twig/variables/FeedMeVariable.php
index 6db1d649..62675ab1 100644
--- a/src/web/twig/variables/FeedMeVariable.php
+++ b/src/web/twig/variables/FeedMeVariable.php
@@ -23,6 +23,7 @@
 use craft\models\Section;
 use craft\models\TagGroup;
 use DateTime;
+use Illuminate\Support\Collection;
 use yii\di\ServiceLocator;
 
 /**
@@ -356,4 +357,26 @@ public function supportedSubField($class): bool
 
         return in_array($class, $supportedSubFields, true);
     }
+
+    /**
+     * Check if the only sources set for a relation field are custom ones.
+     *
+     * @param mixed $field
+     * @return bool
+     */
+    public function fieldHasOnlyCustomSources(mixed $field = null): bool
+    {
+        if ($field === null) {
+            return false;
+        }
+
+        if (!isset($field['sources'])) {
+            return false;
+        }
+
+        $sources = new Collection($field['sources']);
+        $nativeSources = $sources->filter(fn(string $source) => !str_starts_with($source, 'custom:'));
+
+        return $nativeSources->isEmpty();
+    }
 }