So... JUST rn i find out how to make it work
and no its not addon folder or VPK mods it self
It saldy has to be done using pak01_dir!

So what you have to do is -
Download GCFScape
So you can UNPACK pak01_dir.vpk , Edit what you want and its done.

1. Unpack the pak01_dir.vpk and Put it in a NEW folder thats on the Desktop.
2. Edit only Things you want like a Shotgun. Change its texture or add color
3. Save it in the same way as orginal and then Port the ROOT folder in the vpk.exe!
4. Enjoy Retextures / New models!

(if you have more questions . Check out my Tutorial about L4D2!)
(Before that ofc save the copy pak01_dir.vpk and save it in a zip file , just in cake if you get bored of em)