Well, the model needs to be decompiled first (with the Cannonfodder’s MDLDecompiler program, for example) and then you’ll need a 3D program such as 3DS Max, Maya, Blender etc. with SMD tools for importing/exporting, to edit the model by creating flexes for the faceposing. And you have to modify the .qc file of the model simultaneously while doing this.
After you’ve done, then you’ll compile the model again with the Studiomdl, for example.
But… decompiling models isn’t always error free, so it’s recommened to acquire original uncompiled source files from the model’s author if possible.
This is just the basic formula of this “adding faceposing” process.
Hell, I’m not even a Source-modeler, but I’ve just happened to understand basics without knowing anything else how to port even a single simple prop for Source engine, hehe. I’m just a modeler between beginner and intermediate with Blender. But I hope that you got the idea though. (;
Oh, and I have a bit of knowledge how to create flexes (or shape keys as Blender calls them), but as I said, I have no idea how to compile models for Source engine.