How about a "Snowbird" badge for migrating X number of questions to SF or SU?
There aren't badges for "closing questions" because it would cause a stampede of questionable closes. People who are closing things just to get the badge.
Migrating a question is just a particular type of "close", no more or less worthy of a badge than the other reasons to close. It should not get a specialized badge unless each close reason gets its own badge. And, giving each close reason its own badge would just incentivize people to close everything they can see, in order to get the badges.
Closing is not something that should need any incentive. It is a desire to clean up the site that causes us to close questions.
What about people treating programmers.SE as a toilet bowl? Would they get the "plumber" badge?
I don't think that migration in particular needs encouraging. Closing questions in general may be worth rewarding, however.
Would be cool but, playing devil's advocate, might overly encourage the rep-whoring. You gotta get to 3000 before you can (in a trackable way) help to migrate questions to other sites.