It's a video game, you can do whatever you want with it. So I'm certain that a system to allow DRK to spend its HP on attacks is entirely doable. Like attacks could cost both HP, and MP with MP acting like a regulator to prevent things like the player KOing themselves or a healer pumping a DRK with heals for damage. A DRK would need both resources for damage. And then DRK could get a barrier that is equal to the amount of HP spent that has no time limit with each spender attack adding to to the barrier's strength, and the barrier strength could even be displayed as a job gauge. DRK could be given an emergency button that instantly converts the barrier HP back into regular HP.