Uh specular maps in source works. Just keep in mind that they’re a tiny bit different compared to other games as far as I understand.
Your first map is he specular - which is in the alpha channel of the normal map. This controls the amount of the light reflection. And probably works more like a standard specular map in other games. Sadly it can’t be coloured in the current source engine iteration 
The second is the specular exponent. This governs the tightness of specular. (Rubber and metal can be just as reflective but the rubber will usually be softer for instance).
This is a seperate texture and once again uses grayscale values.
If you want something that really resembles metalic textures though, I’d go with two textures. Just give a different texture ID to the metal parts and duplicate the vmt from your base, and assign a phongwarptexture to them. Use the combine hunter as a reference. Just make a grayish white phongwarp texture and it should look pretty metallic.