I support this. Every game needs a zombie mod. All you'd have to do is, like you said, change the dretch model(and disable wallwalk), set the alien team to all AI dretches, and their simple AI would work just fine for a zombie.
You could add more stages to humans(and spread out the current upgrades among the new stages), and as you increase in stages more zombies come to make it harder.
You would also have to change the lighting in the map to create a more creepy feel. If you've ever played the zombie mod for CounterStrike you would know what I mean. The lightmaps are disabled and everything is reverse fullbright(fulldark), so that dynamic lights still light the area when you fire a gun, light your flashlight, etc. You don't really have to do this though because the zombie mod in Counter Strike allows you to play as the zombies, and you utilize the darkness to make up for your sluggish speed. It might still add to the gameplay if everything were really dark and hard to see the zombies. You could also force thick dark fog in the map so your visibility is lowered still.
And bout moding: well i cant help you sorry Sad
Then why do you have that signature?
EDIT: In my opinion the zombies should be the human models, only with all the animations stripped out and new ones put in for better zombie animations.
Or, you could have new zombie models, but have players converted into zombies when they are hit, and the textures on their player models change into zombie-form.
I just made these textures:
(tga)
(tga)And now a screenshot:
I also set u_shoulderpads to gfx/null to disable the shoulderpads in upper_default.skin:
tag_head,
tag_weapon,
u_armour_light,gfx/null
u_shoulderpads_base,gfx/null
u_shoulderpads_light,gfx/null
u_torso,models/players/human_base/base.tga
tag_torso,
