Trying a simple hack and already having issues!

Anoy! I posted here a few years ago before deciding that FF6 was just too much of a bear for me to do anything significant with. But it's still my favorite FF game, and I do want to make small changes from time to time.

So to take an idea from another hack I played who's name escapes me, I'dlike to give Celes a little late game boost in power. In that hack there was a hidden relic that changed Runic to Shock when it was equipped and I'd love to make that and put it in my game. I was thinking of replacing GP Rain since it performed a similar function but is actually terrible. I'm trying to use FF3usME to do it, but I'm having trouble pulling it all together. Is it possible with just that tool? My original plan was to just change GP Rain into a copy of Shock and make it equippable by Celes and Gogo, but I can't seem to find a way to alter the GP Rain 'spell'. It's not in the spell list, or at least I can't find it. Any help on this, or if there's a more effective method of doing this would be greatly appreciated. Thank you!