Couple ways this could be done that I can think of.
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Add a cloak slot that increases hot or cold resistance to a greater degree than most armor does.
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Add an armor modification that changes hot or cold resistance.
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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.