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 vjass

which 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.