It's hard to say exactly without seeing the recipe for caramel corn, but you should be ok using plain microwave popcorn. It's hard to find microwave popcorn without flavorings, salt, or oil, but if you can find one where the only addition is salt, as long as you reduce any salt in the recipe, it should work just fine. The main thing to avoid is microwave popcorn coated in oil -- the caramel might not stick as well.