Depends on the size of the Camelpack. If it's a small one and will fit, wear it under your jacket and heep the hose inside.
I've used a larger one, but even with insulated tube and blowing back it would freeze up when temps start to get below zero. One additional trick is to drink consistently, every 5-10 minutes...the colder it gets the more often you need to drink to keep the tube/mouthpiece from getting a blockage that won't clear easily.
On super cold days, I either don't ski with it, or I just accept that I'm going to need to go inside a couple of times to warm it up and get the water flowing again.