If you can only use the fireball skill of a staff if you could learn fireball itself, why even bother about the staff? If the only good thing the staff gives it the fireball skill, you are better of learning the skill and equipping better staffs.

At current state I think you can't equip staffs during fights to learn and use their skills, because you can't change your loadout during fight. At least this problem with the Staff of Magus Skill 1-2 patches ago.

Also changing equipment still cost those precious ActionPoints, and you are probably most times just better off doing something different with them.