Of course, one cannot create new new spells, only reuse ones from the original game. To do so, create a scroll (you'll find it in the "Objects" menu, and it glows green). Set INFO(0) to an spell ID number:

0: fireball
1: statue
2: ice wall
3: ice wall
4: tree
5: boomerang
6: vblade

IDs 7 through 11 are assigned to spells that went as far as to be named, but not actually to be given structure to; they will not function if you refer to them. (A list of the unused spells can be seen in "Why doesn't X object work in gameplay?", if you care.)