Well an easy way would be to buy tomes/units (with no effect/removed) and well trigger the abilities to be added and leveled up.
The problem is that they need to be unit abilities and those cannot be upgraded on level up.
What most people do, is that they have a default 3 or 4 abilities which they trigger to be hidden/removed and level up the custom abilities for the customized champion/hero.
Really depends on what method you decide to use.