yes, you can backstab (or backsmash) with a staff. In bg1, staff of striking is great for this. 1-6 + 9 damage, and the 9 is multiplied x4. it has limited charges though.
the dagger of venom is another great backstab weapon. doesnt do awesome damage, but the poison is great against mages.
in BG2, you can get staff of the ram, 1-6 + 12 + 1-4. Combine this with a belt of fire giant strength used by an assassin and a critical hit, and you're looking at over 250 damage!!
