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In an alpha channel, you can store a greyscale image that can be used as a mask, or loaded as a selection. When you load as a selection, the selection is available for all layers.
When you ask whether you can add a layer-mask to all layers with one push on the button or shortcut, I never heard of that.
What you can do in that case is create one layer with a mask, and use Ctrl/Cmd+J to duplicate it as often as you fancy (withih PS' limits of course)
I tried and couldn't figure out if it is possible to do all the layers in a batch but you could speed up the operation by creating a two step action.
1. Command (Control) + click the target layer to create a selection
2. Click the Save Selection as Channel button at the bottom of the Channels palette.
If you assign an F-key to the action you could run the action with one hand and use Command (Control) +[ with the other hand to move the target layer down the layer stack.
Welles, I hadn't considered an Action because I was dealing with different layers rather than different documents. Although not "one hit" this would clearly speed things up and save jumping between palettes.
I'm just trying to get some CorelDraw paths/shapes into PS and convert to Alpha channels without doing it one at a time. I have to convert to pings from Draw and I've already discovered I can open all the files as frames in Image Ready in one go, jump back to PS and the frames load as layers, which saves time. Alpha saving the layers with an Action will make the process that that little bit more efficient.
Batch Alpha Channels ??? 
Linear Mode
