No, not currently.

What you can do is teach them abilities instead of spells. The only real downside is they're only usable in tactical combat.

Another work-around is to require a certain stat for casting spells, that only some champions have - like the spells use intelligence now, you can require new made-up stats instead like (UnitStat_IsFireCaster). Everyone will "learn" all the spells, but only those with the right stat will be able to cast them. Sovereigns are exempt from these limitations, unless you mod that too (which is easy).

How do you mean, use them? Unlocked spellbooks just have all their spells added to a single list.

My idea was to create a Full Metal Alchemist mod. Each Alchemist would have a different ability which no others would have (flame, stone, water, etc.)