In the last 90 days, we have migrated one question to Cross Validated (image here for those who do not possess the necessary privileges to see the migration statistics page):
I think we need a history of regular well-received migrations before adding it as a migration path in the "close" interface. From When should we consider adding a default migration path?:
Single digit migrations in the last 90 days and/or rejection rates that can order a drink without ID are signs that the path will be a waste of time to establish.
If you believe a post should be migrated away, vote to close it with a custom close reason and also flag the question for moderator attention and say where you think the question should be migrated to. See this post for more details on when to use this kind of flag and when not to.
Note that we should not migrate posts that are of such poor quality that they are likely to be closed for other reasons on the target site, e.g. questions that are too broad or too vague to be answered in their current state. For those questions, the OP needs to improve the post before we will consider migrating it (see the Don't migrate crap rule.)