There's literally no way to add belts in to the game at this point. In order to have belts on clothing, everything would have to be designed for them and it would massively restrict what kind of clothing and armor designs they can do. Otherwise, there would be massive clipping issues with the clothing for every single belt out there.

WoW gets away with this by making every outfit have a skin-tight clothing model that doesn't move around the waist. When they did have something more lose like a guild tabard, it had a bar missing around it that was specifically put there for the belt to cover.

Having belts incorporated as part of a design is feasible, but attaching it after the fact? Nuh uh. It's the same reason that on some outfits necklaces can't be worn or how sometimes gloves/boots will go over clothing and sometimes under. It's all to prevent clipping and there's no way to change it and still have the game look nice.