Well depends on what you intend to do with the hybrids... There are already a few on the market with cornsnakes, milksnakes and kingsnakes.
If you intend to put them back in the wild after, then you should be shot.
But if it's for your personal pleasure and intend on not selling them offspring or not reproducing them, then you can do it.
Don't forget, you are playing god when you do this.... use with carefulness... I wouln't do it, but if he ever ends up with something I would like to see pics.