Copying the models into the directory and rewriting the .fgd might work, but I doubt it. The weapons are coded into the original OpFor dll and are probably not included in the HL dll. Spirits entities are coded into the Spirit dll, modifying the spirit .fgd probably won't work with HL (although I haven't tried it?).
For more informnation on stuff like this, check Collective and some of the Development sites.