Converting to a Winter Pack, need some modular suggestions for my 3 season pack

This may not be UL, but functional for what you need. OR makes some stuff sacks with a daisy chain sewed in them. You could then lash this to the outside of your pack for overflow storage. I am not shure how waterproof they would be, but just pack carefully with what you put in these.

You could attach water bottles with a carabiner and put other gear in the side pockets. I sometimes do that for winter trips with my smaller GG Gorilla pack. Also, a tent or jacket could be carried under the top flap. A dry sack could be carabinered onto the pack for clothing layers or stove and pot. You could try to pare down your winter gear, using down instead of synthetic insulation (if any).

I think a 55 liter pack should be sufficient for shorter winter trips with the right gear choices. I use a 65 liter pack and have room to spare. not sure exactly what your pack looks like, but the simple addition of some shock cord and a cord lock can save a decent amount of room- things like a hardshell, tarp, sleeping pad, etc- things that aren't effected getting wet along the same lines, Exped makes a nice decent size mesh pocket (I'm sure others do as well) that is for all practical purposes universal in it's attachment

I really appreciate the feedback everybody. Has anyone ever used those equinox pouches? They look like the OR pouches mentioned earlier in the post and are cheaper :)