it depends on how much is severed. if its just one limb they can piece together again but if they are all broken, smashed, severed and spread out its alot harder to piece it back together.
as far as i know the zombies doesnt have excessive regenative properties, (thats reserved for heroes only) they would have to sew their limbs back on. (did we see that zombie actually reattach his head? did we really see the head still working before it was reattached?) imo a smashed or broken limb is different from one that has merely been severed.