I didn’t ski tour much with our little when she was first born (it was bike season by the time I was getting out for longer days) but here is an article from someone who did https://www.outsideonline.com/culture/active-families/many-considerations-backcountry-breastfeeding/

Things I would keep in mind:
-Type of pump. If it’s cold it might not work. If you have something like a Freemie you’ll probably be able to pump more reliably (I know people who’ve used it while biking so image you could use it will skinning at a low pace). And if it is cold at least you won’t have to worry about ice packs!
-Pumping in the car. I pumped a lot while driving (friends got used to having to wait for me to unhook the pump/store milk/rinse pump parts at trail heads. And the reverse going home). This gives you the longest window to potentially go with out pumping.
-If you’re worried about leaking, some removable pads could help (swap them out so they don’t freeze)
– It’s not the end of the world if you miss a pump session. It’s not the end of the world if you miss a pump session. I repeat this as I had a very hard time mentally when I thought I might miss a session.
-Formula. I was a pretty diehard breastfeeder/pumper. But we kept some back up formula on hand as it gave me the space to go a little longer without having to pump.