Seems like an extremely niche use to me, honestly. There's little reason to use possession mode outside of certain user-made custom maps right now, and I think mouse + keyboard fully suffice for these few circumstances. Adding full-fledged controller compability to the game for this alone is not worth the effort, imho.
I would like that definitely try it but say if it's not relatively easy don't bother. Possession isnt that great, not using spells/potions/rituals in any confrontation is a major handicap. It's still cool for arena based maps.