Manual:$wgImportSources

Details
This setting lists allowable "import sources".

It contains interwiki prefixes for wikis from which pages can be imported using Special:Import.

Since complete page history can be imported, these wikis should be "trusted".

The transwiki import interface is available to users with the "import" man>Special:MyLanguage/Manual:User rights|right.

Syntax
  can be a simple flat array that contains valid interwiki prefixes.

This example allows import from Wikinews, Wikiquote, Wikipedia, and an additional wiki that has been added to the interwiki map:

For more complex scenarios, a two-level array can be used.

Sites can be listed as standalone wikis (like  in the example below), or they can be listed as projects with a list of subprojects (for example, the Wikipedia project with Czech, English, etc. subprojects).

Note that the subproject prefixes are resolved using the other wiki's interwiki map – so in the example below,  would be looked up in your wiki's interwiki map, while the subproject prefix (  etc) would be handed off to the   site for an additional redirection.

For even more advanced scenarios, you can use the  hook.