HOW DO I ADD RECOIL DAMAGE TO SKILLS?

What do you mean by recoil damage? What engine are you using?

More details helps us to help you. We can't really give you a good answer unless you tell us what you're after. You just want counter-damage? What is recoil? How does it work in your game?

Demonic Tutor has skills that cost HP, rather than MP? Er, I mean, I'm not quite sure if that's the kind of effect you're looking for, but, it's the first thing that popped into my head.

Yeah, looks like this one has it. Don't bother with the dropbox links, YF's account is suspended. Use the github one. Don't miss that you need the Lunatic Objects script for the recoil one to work, there's a link to it in the recoil script page.