Sounds like it could be a good idea, but unless you have one of the more expensive pepper grinders where the base turns to do the grinding I dont think you could be succesful, since the inexpensive ones that are already filled with pepper use the turning of the top to grind, and since they do this it would be too much fabrication to make it work. Also, I am now sure how much difference the blades in these would make for grinding, since the grinders usually just sort of thickly chop the pepper, and are kinda dull. Whereas actual grinders use friction.

This again is just a quick guess, I am sure some things weren't considered.