that's simple. In data editor, in right to the spells section there should be effects (Or maybe "Buff", I have non-english version so don't know), right? If not, you have a too low game version and you don't even have this function immplemented!
So anyway, if you have it, create a new effect in there, name it as you want and give the icon you want, set so it wouldn't have any sounds/effects if you want. Now with the spell itself, scroll down and find the "ability-effect" or whatever it's called. Anway it should have an "effect" or "buff" in it. Now you change the one there is to one you created and it should be done.