I believe so. I do not think there is a rule that would prevent that. Someone correct me if I am wrong.
only monsters can't join units, not the other way arround. ofcourse there are no units of monsters but as far as i know a character can join a unit of monster and handlers [i.e. hydra]. regarding kroxigors, they're a unit of infantry just like skinks or saurus so characters can join and leave he unit as normal.
The hydra have a specific rule preventing chars from joining hydras, but other monsters and handlers could be joined by a char.