Just make the "PC Zombies" special zombies, like the Infected you play in L4D. This allows you to introduce a class system to Zombies. PC classes are determined by "Profession".
Sample Ideas - Character Classes:
Survivors - Survivor's don't have classes so much as "backrounds" that determine their strengths and starting abilities. Survivor PC's increase various abilities by using them, their profession determines their starting level with certain skills and the rate at which those abilities improve
Street Rat: Stealth and/Or Scrounge abilities
Doctor: Crafting medical supplies
Biker: Engineering Skills/Melee
Policeman: Guns
Or alternativly, you could use a Fallout Creation method that allows Human players to tag certain schools and construct their own "Class". On second thought, this probabl works better.
Zombies - Zombies are more like traditional MMO Classes. They have an archetype, they level up by consuming genetic material - Flesh, Brains. Brains are worth more genetic material of course. This Genetic Material "experiance" lets the zombie level up.
Hulk Zombie: Some type of big, tank monstrocity, super strong, but slow. Cannot use doors or enter buildings without smashing down walls/doors.
Chaser Zombie: Some sort of fast, super agile zombie. Fast but weak, can climb walls, possibly superjump. Think "Hunter" from L4D.
Intelligent Zombie: A weak zombie that still partially resembles a human, but grotesque and/or decayed. Can use tools and weapons and command pet 'mook' zombies in combat.
....hmm, guess I gotta design one now.