That's going to be harder than usual because his shape is very different from the witch's. Your model would have to be rigged to the witch's bone structure to use all of her movements. I don't use 3dsMax so that's also a problem.
If you know how to model and rig a character then you can probably follow this tutorial: http://www.gamebanana.com/tuts/9810, to get an already existing model onto an L4D2 character. Except you'd use the witch's content instead of Ellis', and I think he even goes over 3dsMax plugins to export for L4D(2)/Source.