If an AT is slipping and the fluid is at the proper level, repair is knocking at your door. If you want to try an additive try one of the Lube Gard products, but don't count on a fix in a bottle. Don't keep driving it while it is slipping!
A pan drop and filter might help, I would give that a try after I was certain the fluid was at the proper level, and before I add anything hoping to fix it.
AD