There should be no problem of adding "alpha". By the way for the best result it's useful to add "alpha" to all the textures: 57_2_1_t, 57_2_2_t, 57_2_3_t, 57_2_4_t, 57_L_6_t, 57_L_7_t, 57_R_6_t, 57_R_7_t, 57_RER_t, 57_REL_t, 57_x_5_t, 57_x_15_t.
1. Open the files in "photoshop".
2. Add new channel (alpha). By default it's black. We don't need that (extreme reflections may occur).
3. In "color picker" set this (may vary. The rule is: the darker the more reflective): R:237, G:221, B:221. That's all.
4. Save the .bmps (under their original names, of course) :)
5. Download (if you don't have it yet :() a programm "Imagetool" (can easily be found in the net).
6. Open the previously saved file(s) in "Imagetool".
7. Click "Save as". Type the file name and in "file types" choose "bitmap".
8. Enjoy the flight.