you cant.. theres a hugh code in vjass that allows that, but it requires a hell load of abilities,
and 1 hell of a code..
and its vjasswhich i supose you dont want to use..
and besides, ,another way is to use item abilities, but it will be bad later, cuz you'll have lots of spells, item abilities are inivislble spells that gives buffs (dmg, str, life, move speed, attack speed, etc).. just make dummy spell with all levels of the spell, and add it for X time, then remove it later.
Well, you can use the item health bonus ability.
But you wouldnt be able to change the health difference much because you would have to create one ability for each bonus amount
You can use Item Life Bonus as base ability to increase a unit's HP
BUT, the increase of HP is pre-set in Object Editor, not by formula in Trigger Editor like:
Bonus HP Gain = STR * 100
Test map attached
EDIT:
Dang, am I just rephrasing what wolfman said ?
If i understand d4rks method correctly, couldnt you do this using only 2 abilities?