It's certainly possible, in general, to add extra migration paths. But we only do so when there are really a lot of questions being migrated along that path, enough that it starts to tax the mods' ability to handle them case-by-case, and when the questions being sent there are well received.
Basically, we're trying to avoid the situation where people regularly vote to migrate questions to a site when they don't belong there. It's straightforward to avoid when only mods are migrating to that site, because it's a limited group of people and we know from experience how to respect the other site's scope, but when you open up that ability to all close voters, there's no telling how much care people will take to vote to migrate only what really should be migrated. Hence the reluctance to add new migration paths unless they're really needed.
At this point, I haven't noticed enough questions being migrated anywhere other than Math, nor flags/comments suggesting so, that I really think it's necessary to add a new path. I don't have exact statistics at the moment but I can tell by looking at recent examples that migrating questions anywhere other than Math is pretty rare, which also supports that.
What you can do, for now, if you think a question should be migrated: first, make sure it's off topic here. We shouldn't even be considering questions for migration unless they're off topic for us. Then you've got a couple of options:
Just vote to close as off topic, and you can leave a comment telling the asker that their question may be better received on another site. I use wording like
I think this is off topic here, but it might be better received on [site]. You have the option to delete this and post it there, but do check the rules on [site] to ensure it's on topic there.
Bear in mind that askers can't delete questions which have upvoted answers.
Flag the question with a custom mod flag saying something like
Off topic here, possible candidate for migration to [site]
You can do either of these or both, as the system allows. Personally, I think it's often better to suggest that the poster delete and repost, if they can do so; migration is more useful for when there's existing content like answers that is worth preserving on the other site.
