Mods, is it possible to make them for any game?

[1996 Times]

Games like sofII and quake III has mods, mods that were created by im guessing the fans of those games, maybe even the creator of those games. I was wondering is there a program where i can create mods and add-ons and skins for any game(pc games).

Ps, if i posted this topic in the wrong section, please move it. If this post is in any way harmful to anyone, meaning illegal forgive me for asking.