Specifically I'm trying to edit this mod: https://www.curseforge.com/minecraft/mc-mods/naruto-anime-mod-plus/files/3050483
I'm not just trying to edit the config, I want to add custom stuff as well.
You can't do it easily because the mods is NOT open-source. In fact, it's All right reserved, so you are not allowed to do it. So do at your own risk.
You can ask to the original dev, but it's sure it will decline your question.
Else, you have to decompile the jar thanks to utilities like javadecompilers.com.
Create a blank java project
*The code obtain thanks to decompilation
Finally, I think it's a better idea to create a new one, specially because it will be faster to upgrade it in 1.17 or make more edition.