While trying to import special characters i've found an easier way with DxtBMP and Gimp:
- Open DxtBMP and the input tga file
- Send the main image to paint
- Save it as a bmp
- Send the alpha to paint
- Save it as a bmp
- Open the main bmp with gimp
- Open the alpha bmp as layer
- Play with opacity on layers to check the alignment
- When you done with modification, make one of the layer invisible and click on the other
- Select all the image
- Paste as a new image
- Make the same thing with the other layer
- For the alpha layer, convert it to greyscale (Image > mode > greyscale)
- Save both images as bmp
- Open DxtBMP again (but not your tga)
- In the main pane, use "Load extended 16bit image" to load your modified main bmp
- Import the greyscale alpha bmp with "Import alpha chanel"
- Save the result as tga and you're done !
This is longer to write but much faster in the workflow !
Thank you for the initial instructions !