Oh god... of course it can, all you need is the Importers and such.
Also Gmod/tf2 will need a MDl decompiler, and a MDL compiler, aswell as someone esle with 3dsmax or somthing that can make the Phys file for the model.gmod/tf2 stuff, I do it all the time. also you're probably better off with 3dsmax I suppose, but I`m guessing you dont have the money.
but 3dsmax would be better... Most moddelers for the source engine use 3dsmax.
Also here is a small little very good tutorial on how to make quick model edits
http://www.fpsbanana.com/tuts/6909
I myself use the tutorial.

read it atleast 3 times.
PROPERLY too. or else you'll be need to go back to it every day.

No doubt'fully the simon cowell of the fpsc section :S