You can do it a few way's and it really depends on weather the aircraft is made to have reflections. Simply, go to ALPHA, "create alpha template", double click on the alpha and save as a normal .bmp format. Then, edit that template as what you want to have reflective, Black=is fully reflective w/trans, Grey=semi reflective, w/out trans white=High gloss. Once you have done that open in DXTbmp and save as DX1/DX3, and I like to save without MIPS.  Open your original texture file, and then "Alpha", "Import Alpha Channel", Then export "Alpha Channel" and specify any name. Then save your original texture as DX1 or DX3, place both textures, "original" and "Alpha", into your texture folder, That's all. Reason why my method includes keeping the alpha separate from the original is just in case if I want to edit the reflection elements. -Hope this helps, it's relatively enough info, should help, or give ideas.