Left 4 Dead 2 Zombie Texturing Difficulties

So as most of you probably know (if you own the game), L4D2 produces zombie colors procedurally, which is a good system if you want to save/preserve resources, but I don’t know how on earth I can go about editing the base texture (the alpha channel in the cim_head_shaggy.vtf file) without messing everything up. I’ve tried using VTFedit so I could get the base texture out, and I’ve succeeded at that, and edited it to my liking. I’ll include a picture so you know what I have so far. But after I’ve got this texture, I’m stumped on what to do next. I’ve tried adding an alpha channel then adding that to the existing alpha channel of the zombie texture (the zombies textures in L4D2 work in channels instead of layers, read this if you’re curious: http://www.valvesoftware.com/publications/2010/GDC10_ShaderTechniquesL4D2.pdf) and that didn’t work.

Here’s the pic of the texture I’m trying to apply:

So where do I go from here?