Technically, no you can't. If you add reducer or transparent base to the opaque color, you're really just extending the amount of spraying it takes until the paint reaches full opacity. The difference between opaques and transparents is that when opaques reach full saturation, they will get no darker, no matter how much you spray. Transparents will continue to get darker and darker the more you spray until theoretically, they become black. Now in reality, this doesn't happen with light colors like transparent yellow, but you get the idea. Also, the pigment size in opaques is usually larger, so lets say you spray the opaque at 50% intensity, it will not be as transparent as a true transparent color sprayed at 50% intensity. What you can do is buy mostly transparent paints and you can make them opaque by adding a small amount of opaque white. This is called buffering. Read Dru Blair's color buffer theory for more info on this. This way you can stock mainly transparent paints and make them opaque when needed by adding white. The only color that can't be buffered is transparent red. If you add white to red, it will turn pink, so you have to buy opaque red. Hope this helps.