With the right winter backpacking gear, you can be warm, dry, and comfortable snowshoeing or hanging around your camp in the snow.
These tips just concern winter gear basics and are not a full gear list for winter backpacking.
Keeping your feet warm in PNW winter doesn’t require heavy Mukluks. Your 3-season boots might even work, but only if they are loose enough to allow for warmer socks and still have plenty of circulation. I usually use thin liner socks with heavy outer wool socks. If your socks create a tight boot fit, then your feet will be cold and you are better off getting another pair of boots for winter use. Circulation is the key to warmth. If you buy boots for winter, be sure to try them on wearing liner socks and thick wool outer socks (the thicker the better). Even with both pairs of socks on, the boots should not be tight. Look for boots that are waterproof, but breathe, and sturdy enough to use with snowshoes.
If you want to use your summer boots, but the fit is tight, try relacing them, starting about half way up. How you lace your boots is very important for correct winter fit. Even with boots made for wide feet you may need to skip the first bottom eye-rings to get the perfect loose winter fit. More here: https://winterbackpacking.com/how-to-keep-your-feet-warm/