Here's the idea:
Tapping attack button would execute the normal actor attack, but if the button is held down for 1 second or more and released the actor executes a different attack, or a skill that the he doesn't have. Optionally it could be further expanded to use different skills for different weapons.
Personally, I could make good use of a script like this, but I'm just putting the idea out there if anyone would like to have a go at it.