Add Hero Ability Trigger HELP!

I want to make a trigger where if a unit enters this region add ability to hero

For instance Banish

I dont want it where unit-add ability because you can level up your spell

i want it so that you can add ability and when hero levels up you can level up your ability

Im making a Hero Arena map where you can choice your ability

Event
Unit - Enters Regoin

Codition
Entering Unit is Equal to Wisp

Action
Kill - Entering Unit
(and some trigger to Add Hero Ability)

It cant be done the way you think without massive trigger based systems that replicate learning, multiple abilities for each one, and masses of upgrades.

I can only hope blizzard adds support for this, if its possible.