ImportSources are currently defined per wiki in InitialiseSettings, and if nothing is defined, Special:Import doesn't work.
It would be nice to have a reasonable default for this, so that small wikis would get something sensible if nothing explicit was defined. English Wikipedia, French Wikipedia, and Meta could probably be a good default import source.
I'm mostly concerned with Wikimedia sites, but this may be useful for all MediaWiki installations, so perhaps this should be defined in core. The current default is . But if there's no robust way to do it, then such a default should be defined at least in the Wikimedia MediaWiki configuration. Hence, tagging both MediaWiki-Export-or-Import and Wikimedia-Site-requests.