You cannot. It's not worked for years now, since the new NPC script system. Clarr was the one that was planning on reimplementing all those funky NPCs, but hes gone and the other coders haven't had time to look into such an insigificant feature. Abyshen (whatever that bugs name is, hehe) did start at it, and they could for a time injure you, but I'm not sure on the status of that.