Hand warmers might provide a little bit of warmth, but won't keep a sleeping bag super warm on there own (unless you use a handful of them!). A better use for them would be to either use one to prewarm your sleeping bag, before you get in it or to use a hand warmer down by your feet to provide a little extra warmth on a cold night. However, the warmth will be fleeting, because hand warmers only last for an hour or two. Note: make sure you remove your socks when sleeping in a sleeping bag (unless you want cold feet).
If you're really worried about staying warm in a sleeping bag, you should consider getting a top of the line sleeping bag from FeatherFriends.com