@TerraNexx: Would check for level > desiredLevel - 1, because you might skip levels. The Learn Skill action accounts for selecting a hero ability from the hero ability menu, it was said that the skill would be a normal unit skill.
@Boykillah: Then input some requirement for your ability in object editor and fulfill this requirement via trigger. Requirements are player-specific though, so if a player can have more than one hero with this ability, it won't work. Maybe in this case establish some dummy skill for the hero that gets replaced in time and write the requirement in the tooltip of the dummy ability yourself.