This is controlled via .txi parameters. Without looking at the file, I'd guess the mod uses the parameter "envmaptexture CM_Baremetal" to make certain parts of the texture map shiny. It's essentially the same parameter that gives the sith troopers their fake reflections.
The parts of the texture affected by these paremeters is determined via the image's alpha channel. Basically, you would use an image editing software (e.g. Photoshop or Gimp) to paint out the areas of the image you want to be affected by the parameter.
You can learn pretty much all there is to know about .txi parameters here: