Hehe, that sounds like something Fuuma would be interested in. :3
Kamui had such resistance to getting the kitten in the first place, I think it'd be impossible to get him to willingly adopt something else. BUT. Provided someone was there to enable him... he might go for a bird. It would have to be something that would love him back interact with him, so a fish or reptile or something like that wouldn't work.
Who in the prison, aside from CLAMP-canonmates and Soubi, does Fuuma find interesting?