If you have image magick then you can batch up a command to do this by saving out the color and the alpha as image_c.png and image_a.png and running a batch file with this in it...

composite %1_A.png -compose copy-opacity %1_C.png -depth 8 %1.png

where you pass in 'image' as its parameter and it converts it to image.png which is the RGBA version.