As it turns out...
I'm a prosthetist and a snowboarder, so I should be able to answer your question. However, that's kind of a tough one to align without a visual. Intuitively I'd say:
-widen your stance to disperse force through the knees,
-use whichever stance (reg or goofy) that causes the least stress on your prosthetic side knee,
-use supportive inserts (ie Superfeet) to enhance medial support and promote varus alignment,
-Bring extra prosthetic socks so you don't go home with an angry residuall limb. Or use a thicker gel liner for more cushion when riding (if you wear one).
Ideally you would have a snowboarding prosthesis with an exaggerated inset of the foot to promote varus at your knee. But insurance rarely covers this sort of thing. Maybe next time you get a new prosthesis have your prosthetist turn your old one into a snowboard leg. Additionally you might be able to customize the cant on the footbed of your prosthetic side binding so that you have an angle that's high to the medial side to control some of that valgus.
It will be a give and take experiment. The body will only allow so much postural deviation off of how it has wound up to this point. Good luck and have fun.