If you mean reflecions, use alpha channels on the regular images. (T#) the closer to black in an area, the more reflection that area will have. the closer to white in an area, the less reflection there.
If you're talking about specular shine, add an alpha channel to the specular image the same way as to the regulars. (T#_spec)
And save them as a Extended Bitmap DXT3 or DXT5.
FSX and FS9 both read those quite well, and I've heard if you save as DDS, you may have to deall with all sorts of like flipping issues or something.
Hope it works!
If you didn't understand all of the above, export the alpha channels from the original and save them as a bitmap (if the originals were DDS, you'll probably have to flip it vertically before you save). Then open your textures, and import the alpha channels and then save as Extended Bitmap DXT3 or DXT5.