Welcome to FEU! I see you’ve found an old thread. In the future if you only have a question, you should just start a new thread in the Questions category. These old documentation threads are left open just in case someone wants to add new info they’ve figured out about it. Not to worry, though!
Regarding your question, I don’t know how for fe7, but Shuusuke has a small hack for it for fe8.
This section in particular:
0802BA0E, the default value there is 0xC0, meaning only blue units can gain exp. To make it so all units can gain EXP, change it to 0x0 like in the image. Change to 0x40 to allow only NPCs to gain EXP, or 0x80 to allow only enemies to gain EXP
Also important is to not forget to set the units you want to gain EXP to 0 (or just a value of not 255, since that is the comparison)
If you are hacking fe7 to make a reskin, I don’t have an answer for you (this thread says fe7), but fe8 is the standard to hack in general.
Hope this helps! Good luck with the hacking.