a normal spell that is learned by a hero at level 5 or above? you can just add it via triggers....
btw, normal skills dont have a level requirement tab right? maybe you placed 6 in the Techtree- Required Levels tab in the object editor...
if its what you have done, then I shall tell you, the level you place there is the level that a certain techtree upgrade level should be for you to be able to use the spell example(you add the techtree requirement in another tab, I think its above the level)... for example you can try searching through the human priests abilities......