I'll start with the bad news, if you just give Tormod the Lumina skill he wont be able to use light magic, You also need to modify his weapon levels so he can use staffs and light magic.
The goods news is that i know how you can do this in path of radiance, but not the way you're trying to do it. I did it using the "battle debug menu" action replay code. I can post the code here, and tell you how to do it if you want. But i only have the codes for the PAL version of the game, so the code wont work on the NTSC USA version.