No "hur dur." Don't be insulting. When you are talking about something that violates the physical laws of the universe....by definition you can no longer rely on the physical laws regarding that phenomenon.
No insult intended, just stating that magic is stupid.
I get that it would have to involve magic, but would the addition of cells be traumatic, or even lethal?
Okay. Sorry to jump there. But by what method are the cells being added? You are talking about the creation of matter from nothing. Since the method is magic, there are absolutely no comparables to base an educated opinion on.
I could only guess using growth as a comparison. You would experience excruciating pain, comparable to the bone pain in growth spurts, compress to the duration of the generation of the limb. Probably enough to render you unconcious, but not enought to kill. But that's way out there in speculation land.