Stackable means that can be changed ? If not, you may do a trigger like this one. I know, its noobish and needs 2 triggers AND it uses a ability slot, but it might help you.
~1st (Trigger Condition - Trigger Initially on)
Events - Unit starts casting ability
Conditions - Casting ability equal to <your ability>
Action - Add <a ability that adds damage> to casting unit
- Turn on <Trigger 2nd>
- Turn off <This Trigger>
~2nd (Trigger Condition - Trigger Initially off)
Events - Unit starts casting ability
Conditions - Casting ability equal to <your ability>
Action - Increase level of <your ability> (this to increase the dmg)
I could have given you an ability that adds damage and its passive, but i cant think of anything right now ....