Yeah, that's all you could do. Enemies aren't constantly loaded and aren't stored the same way as party characters even when they are loaded, so you'd have to clone their stats, replace sprite sheets on the disc, add a bunch of frames to the sheets for animations they don't have, create portraits for if they talk, and on and on. It's not something a GameShark can do. The only exceptions are the handful of characters like King Jowy that are technically enemies but exist in the same format and space as potential party members for story reasons. You can theoretically put Yuber or Rowd in the party with a GameShark code, and then most likely crash the game when they're not complete enough for some story sequence, or they don't have an attack animation.