You know, that would have been in the FAQ if I hadn't conveniently presumed everyone already knew it -_-. There is a maximum of 5 hero abilities that can be given to a hero. You cannot cross that limit in any way, unless you use spellbooks or unit spells.
Its hardcoded. Blame Blizzard, we asked them before the retail was released to lift the barrier up for TFT. But noo000ooo, it didnt change. Same with buff creation etc. They quite limited everything. Nothing to do unless you trigger it and stuff, which i dont know jack about it