How to create Alpha Channel automatically?

I have a folder containing about 100 TIFF files, each containing an intricate shape against a transparent background.

I need to create an Alpha Channel in each one which replicates the transparent background.

Is there a way to do this "automatically" with Photoshop 7.0.1 (the only version I have access to) under MacOS X 10.4.11?

Record into an action and batch: select layer transparency, save selection as alpha channel. Watch your matting though, as it sounds like these files will be flattened and any feathering in the alpha channel will leads to a fringe of [white? black?]. You could tuck in the alpha channel mask a bit before you save. It depends on the final usage...