[For 3DS Max]
You'd have to extract the .mdl files (and reference files) of the model from the .gcf using
GCFScape.
Next thing, open up the .mdl file you extracted in a text editor. On the first line you'll see IDST followed by a fifth character. Change the fifth character to a comma, then save.
Now open up
MDLDecompiler (this needs to be in C:\Program Files (x86)\Steam\steamapps\username\sourcesdk\bin\ep1\b in to work properly). Select the I/O directories, uncheck all boxes, then decompile. These are the files it dumps:
The main model is the non-lod reference at the top. So we'll use
this 3DS Max .smd importer.
'Nother thing. Valve has the .smd files for character models available so you don't have to go through this awesomeness if you're trying to make a hat or weapon or whatever. They're here (in the sub-directories): C:\Program Files (x86)\Steam\steamapps\username\sourcesdk_content\t f\modelsrc\player