I know exactly how to SOC this, only it can't be done yet.
What you have to do is make every sprite name of the states the player sprites, and make each frame go up from 34 (as in, the first sprite would be frame 35, the next 36, etc.). Then re-name the super sonic sprites to "PLAYc0, PLAYd0, etc." Then you have to put this SOC in your character's OBJECTCFG/MAINCFG, and then put in the extra sprites. The problem is that they won't spawn for your character unless you're a skin with HMS123311 on SRB2JTE. :/
EDIT: Do not re-name the sprites in SRB2.SRB/WAD, instead save them and load them after an "SS_START" lmp in your WAD, and then put the super sprites in. This way, sonic transforms without the game Sigseving.