Is it possible for users with 3000 reputation to vote to migrate a question to another SE site? If not, is it possible for ♦ moderators to do so?
For 3k users: Not yet.
Migrating between sites has been supported in the engine for a while now, but is currently only enabled for the trilogy sites and SE sites that demonstrate a need for it. This is the off-topic close dialog on SO:
We have a similar dialog, but the only option we have is a one-directional migration from the main site to meta:
For ♦ mods: Yes.
On the rare occasion where you do see an off-topic post that belongs on another site, vote to close as off-topic and flag for mod attention. Mods can migrate the post to any SE site (including betas). If we start to see a pattern of migrating to the same site frequently, we'll try to get a migration target there so 3k users can handle it directly