Can a crew captured via the Possession ability be transferred to a friendly ship of the same nationality and use their abilities on the second ship? The Code entry for Wraith makes me think not, but nothing in the Possession ability forbids it.
Transferring the crew is allowed, but all of the extra effects caused by it will cease as soon as the possessed crew is separated from the possession source. It will revert back to its original nationality, it will take up cargo space again, and its point cost will apply normally. (ADDED: It does not return to its original owner.)
The same happens if the Possession ability is cancelled, except in that case you may end up having to eject some crew.