Last year I was planning on substituting skiing for some of my runs, but ended up stopping running entirely because of shin splints so they could heal. So I skiied pretty much 100% of the time during the winter months.
This winter I'm making better progress with my run form (and shin splints, yay) so my plan is to keep running at least 2-3 times a week and ski the other days if there is enough snow(and those ski days would replace scheduled running and be similar in HR zones/duration/etc). I'm pretty sure I'd read here that you could substitute some skiing for runs in a training plan.
I wouldn't just ad the skiing on top of the scheduled running because your training load could get really high otherwise!