"I opened the bitmap with DXTBmp (as normal) and saved it as extended. I guess this is the phase I did wrong."This is correct (you got it wrong)
Backup or duplicate image files!
Remember: you're dealing with the Alpha Channel
Use the Alpha Channel Icons below the Alpha Channel window (top righthand side of DXTbmp screen)
Be sure you have Photoshop (PSD) set in DXTbmp as Default Editor and simply click the first and second icons to move it in and out of DXTbmp
Once the BMP(24bit) has been sent to PSD and edited (S)ave it from PSD as 24bit (16bit and 24bit are the only options offered)
Using said Alpha Channel Icons in DXTbmp, import it back to DXTbmp and send it back to PSD (using Alpha Channel Icons, again) where it can be saved as an 8-bit.BMP
as the trans.bmp Alpha Channel image.
I have not tried it on the actual FS9 B744 as I do not repaint defaut FS9 planes. Thanks for reminding me why I don't waste my time repainting them.
Reference cited: Martin Wright's DXTbmp(x) Page(s)