I haven't played around very much with alien hybrids but if it is possible then clicking on the sim and choosing 'disguise self' should make them switch into 'normal' sim mode. You should also be able to take them into CAS and change their looks there.
I'm thinking that maybe not all hybrids have the ability to have a normal disguise though, but like I said... I really haven't played around with hybrids very much so this is guess work.