i recommend you not make that ability unless you have a low level caps.
either way, you make an ability which doesnt give you an icon. like item spells. this might not be the only way, but i have only thought of this one.
make 33 levels, each level adding 3% more lifesteal. so at level 1, your life steal will be 3% and at level 7, 21%.
then everytime the hero levels up, you check the level and set the level of the spell.
however, that spell is only for a single level. it would take alot of effort to add high levels. i recommend adding a caps for higher levels instead.
e.g. skill level 1: 10% life steal at most, skill level 2: 20% life steal at most, etc.