Couple ways this could be done that I can think of.
Add a cloak slot that increases hot or cold resistance to a greater degree than most armor does.
Add an armor modification that changes hot or cold resistance.
Make alternate versions of each armor type (or at least one set of each weight and stats) with the opposite temperature resistance.
Point is, I frequently find myself looking for armor sets that might be useful, only to be forced to completely disregard them due to them having the wrong temperature resistance.