That's... difficult. If you can find an *.mdl decompiler program this will allow you to remove the mesh data from valve *.mdl filesm creating *.smds and a few other filetypes.
AFAIK there aren't really any decent ones around. Certainly I have found one in development at these forums but it doesn't seem to have been released yet. You might also want to try CannonFodder's tools (Google them).
Once you have that sorted use some kind of a graphics program to edit them, such as 3DSMax, Cinema4D, or Blender (which is free and has plugins for exporting *.smds, they probably work for import too, But I'm not sure). Then you are finished! Yay! Was it worth it?
PS If you do find a good mdl decompiler, please tell me as I am looking for that sort of stuff too.