Very good question. If so, that means that we can, in fact, create creatures at workshops.
No, i just tried this, with the artificial bees mod, tried to turn bees in dwarves, didn't work. But, they did gain a Caste (Honey Bee => Honey Bee Worker) which i'm sure we couldn't do before. Also, it might be possible to turn one type of vermin into another.
It seems there's an insurmountable wall between "vermin" and "creatures" in the code.
But we can change caste / species of any existing "creature" at will.