Hi
I don't think you can do it, but I think it worked for me once.
1. Open up the 32bits .tga skin
2. Go to Load from alpha channel
3. Select alpha channel 1.
4. Select the 0, 0, 0 black in the greyscale palette.
5. Use the paint brush to paint over the part you want to skin.
6. Save to alpha channel
7. Make sure the .tga is uncompressed in options save & exit.