How do I edit the alpha channel?

["maspeir" <maspeir@xxxxxxxxx>]

I am trying to use GIMP as a sprite creation/editing tool. I'm working on recreating an old Macintosh game that my dad was developing in the early 90's. The sprites he used were 8-bit color and had separate 1-bit masks (black and white). I've converted the images from the Macintosh PICT format to PNG and opened them with GIMP, but once I add the alpha channel to the sprite image, I have no way of adding the black and white mask image to the channel. I've tried everything and what documentation I could find on GIMP's site doesn't actually mention how to add pixels to the alpha channel.

Without the ability to actually add anything to an alpha channel, the feature is useless, so I know that I'm missing something or doing something wrong. I really need to merge these files into a single RGBA image.

If it matters any, I'm using GIMP in Windows.

Any help would be greatly appreciated.