I don't unnerstand why you'd need nother spell.There is no National Limit on Djinns; I jus checked the pedia on it. If there's some other reason, plz explain it to me.
Unless there were serious drawbacks to djinn-ifying, they would be unbalanced in the form I mentioned - essentially an unlimited supply of archmages. That only holds if the djinns are allowed to cast the spells they had as archmages. If you stripped the spells, then yeah, there wouldn't need to be a limit.
If you did strip the spells, though, you'd have to make the Djinns more powerful somehow to balance the loss of an archmage. Personally, I like the idea of Djinns being able to cast spells they already know but not learn any more.
Also, because of the increased strength, I don't think they should get any more free XP. To be honest, I don't really know how the spell system is handled, so I'm not sure if they could keep the casting mechanic while losing the ability to learn new spells and gain free XP. But, if the firebows can do it......since the civilopedia says that the only reason they are not extremely powerful casters is that they won't devote the time to study what they could do with all that mana they can channel.