The community migration list (i.e. the list of sites where question can be migrated without moderator assistance) only contains a small handful of sites of common migration points for that site. To add a site there, you need to post on the sites Meta and get community consensus. Generally sites are only added there if the migration route from site A to B is very popular.
A moderator can migrate to any site in the network:
So, what you want to do is close with a custom comment, along the lines of:
This belongs on site x because...
Then you can flag for moderator intervention and the moderator will hopefully migrate it to the other site.