I dont know about other brands, but PPG uses a product called IIRC Flexed and flat. Its a clear product that you add to your un-catalized clear before you mix it. Something on the ratio of equal parts clear and Flattening agent. They also sell a premixed flattening clear that you catalyze and spray however that does not allow you to adjust the ratio of how flat , semigloss, or satin you want the finished product.
I would NOT recommend randomly mixing anything with a urethane clear.