yes, but I think you'll need some excellent photoshop-'skillz' if you want a decent result
don't have much experience with it, but try googling it, I'm sure you'll find a good guide
If I remember correctly, someone made a TexMod package that changed how critical hits looked in-game. Basically, it allowed you to see your criticals hits easier by adding the word "BAM" to it.
I have looked for a guide, but I can't seem to find one. The part that baffles me is how to select the texture for the spell effect in the first place.