This could be similar to the peace talks agreement. Say a tribe was raiding you and lost some raiders, and a colonist was kidnapped. You should be able to bring the bodies of tribespeople back to them and get the colonist's body back if they died after being kidnapped, and there should be an opportunity to exchange prisoners for prisoners or pay ransom.
you already are able to get your lost pawns back for ransom.
but it would still be nice to be able to exchange prisoners. would make it worth keeping some prisoners around.