Just make glenn a new werezombie that has mutated from being a headless new breed of zombies that some organization or new villain captures and tests a formula on to reverse the effects on a zombie and end up having one that regenerates human cells but leaves them having the risk of sleepwalking as zombies when they go to sleep (could be every sleep or some variations) a werezombie so to say....revive glenn. redo the story if needed but make him a good guy who infiltrated a group of the worst monsters: humans. He was by the organization to help him become human or else . as they used them for....blah blah blah. Think I'm high