Thats not possible afaik since the tuning-system is implented into the GTA SA game. The beer you can buy at the Shithole bar is implented by script so they can use it for other things, like earnings. Correct me if im wrong, but im quite sure im right. It can be only possible if LW's scripters would make a complete new tuning menu which they can connect to the earnings-system-thingy.