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*** Bug 49545 has been marked as a duplicate of this bug. ***

It's actually already possible to create interwiki links by including the interwiki prefix in the link editor; example with wikitech: https://www.mediawiki.org/w/index.php?diff=729932 .

Interwiki links that have been created this way with VisualEditor can still be edited the same way as long as the page isn't saved.

Since this works for new and newly-added interwiki links, I'm not sure why they're blacklisted / green-striped in subsequent edits.

*** Bug 50889 has been marked as a duplicate of this bug. ***

I just tried adding a link with [[:w:eo]] and got this:
http://www.mediawiki.org/w/index.php?title=Language_portal%2FWikimania2013%2FEvent_Report&diff=768410&oldid=768371

[Changed the title to avoid confusion with interlanguage links.]

This is kind of possible now, by creating external links.

As Roan says, external links are now silently converted to interwikis when appropriate by Parsoid, so this is totally possible, and has been since we fixed it in November.