[SCRIPTING] [RMMV] MP DROP AFTER ATTACK

What I'm looking for is a sort of MP cost for a action, where the action can be used regardless of whether you have enough MP left, but it will still drain your MP. For example- I have 2 MP left, and I want to use a skill that drains up to 3 MP when you use it. I would still be able to use the skill, it would just would drain my mp to 0.

To put it in perspective, I replaced the MP bar with a "stealth bar" in my project- I want the character to be able to fire a gun which drains 3 SP (stealth points), but I don't want to require him to have 3 SP stored up in order to shoot.