Yes, you can adjust the alpha in a TGA file if you have a graphics program that allows that. Both Photoshop and the Gimp will allow that. You create the alpha channel and then can draw on it as though it were a 256 color grayscale.
GL_SRC_ALPHA, GL_SRC_ALPHA will give the same amount of color from both src and dest, the only thing you are changing is the intensity of the resulting pixel.