Going off of this, I don't think Nobodies would be appable at all, considering that they're pretty much just an alternate of an already existing character.

Replicas on the other hand, at least in the case of Xion, would be because they carry a more unique set of ability and personality. She would be made human and retain her and Sora's memories, depending on what canon point you'd be taking her from. I apologize if I somehow got something construed; Kingdom Hearts has always evaded me.