Exactly like Ayvah pin-pointed, you don't have to put any point in anything to learn any skill.
You get a Fireball skill book? Right-click and learn it with whoever you see fit.
If you give fireball to your warrior, sure he/she'll be able to cast fireball, but 0 point in Pyro means that you won't do a lot of damage with that fireball. You'll still be able to cast it though.
