FF6 with FF5 job system, NEED HELP

You could upload as many sprites as you want to a blank/expanded ROM, but it's meaningless if the code isn't setup to place those sprites in the right place in battle/on the maps.

If your next suggestion is "So setup the code to do that!", then I would mention that it would be faster to write your own FF6 battle engine from scratch than to rebuild the code for what you're suggesting.