Places like REI and North Face make a special half-length bag called a bivouac sac or a "bivvy bag." Meant to be slipped over your legs while your upper body is in your down parka, it is a small item to help you if you have to stay out overnight on a cold climb when you weren't planning to camp over.

That said, it is a lot of expense or trouble to go to--around here we all have found that flannel sleep pants are great, very comfortable with the usual jersey t-shirts we wear to sleep in. The local Macy's had some for sale in the men's department last year in the late fall that were quite thick, very nice. Add a pair of non-skid socks. You could re-assign a pair of regular socks and use fabric paint to add a non-skid pattern to the bottoms. That's what we did when the kids were little. Get the puff paint type, and wash them inside out to keep the paint in place longer.

This way, you're reusing, you're not adding to the clutter in your sock drawer. :)

SRS