Add your own spell by creating a map mod.

I was digging through the menu settings and found a "spell IDs" option, and I saw that you can add a new spell ID there, but all the spells are already taken, and a new one would be SPELL_NONE. Now, based on this, is it possible to add a custom spell to a map, making it a mod? Let's say it's a modified spell like "Lightning" or "Fireball," with altered textures and damage or range, but so that it's another spell in the spellbook, and the original spell remains unchanged. Or is that completely impossible?