Yes... I am afraid adding more spells is out of the question as well. Past BF it is AI commands of sorts that shouldn't be touched, from what the Babil Documents say.
That said there are a few blank spaces for you to use...
The Spells right after W. Meteo are unused, DullSong is unused, (It's part of a dummied Counter sequence for Milon-Z, but it is never used) The spell right after "y" is unused. B9 is unused, and BD is a super-haste Effect, but it is completely unused.
So right there that is 6 free spell slots... but if you meant Actual spells that the player can learn.. then I am afraid that is a bit trickier because of the 5 character limit and the 8 enemy spell name issues.