Manual:$wgImportSources

Details
Array of interwiki prefixes for wikis we'll accept as sources for Special:Import (for sysops and appropriate permissions). Since complete page history can be imported, these should be 'trusted'.

m:Help:Help m:Help:Help Interwiki Interwiki

If a user has the 'import' permission but not the 'importupload' permission, they will only be able to run imports through this transwiki interface.

Example
This is an example of what to add to your LocalSettings.php page. You can change the values of the array (wikinews, wikiquote, etc.) or add more entries as necessary: