I'm a noob at this stuff so bear with me. I am wondering how I to make the Unter (axis sergent) wear a different hat, but I still want to keep the old hat for other classes. The new hat would be hat2.bmp?

Is there a place where each class is "mapped" to the various ".bmps"?

I've been playing around, teaching my self the basics. I've managed to compile, decompile, and fix the qc directory pointer.

so, you've decompiled the model into a directory. . .

open mp40.ref or whatever class you'd like to edit.
click the group tab, highlight helmet, click select.
from here, you can delete the helmet.
click file, import > HL smd, import the headgear.smd (triangles only, no skeleton).
get the headgear into position, save mp40.ref.
compile model.

btw, the medium class axis looks pretty cool with no helmet at all.

if the headgear is floating off to the side of the model after you compile, you may have to assign the headgear to a bone, right?

for more on that, let's pass the mic to . . .

I suggest finding a modelset that has this and PM the author. It's been done before, but I don't know how, unfortunatly.