If you’ve got your roto’d footage on a layer and the background is transparent, Command/Ctl-click on the layer’s thumbnail in the Layer palette to make a selection of the opaque pixels of that layer. Switch to the Channels palette and click the “Save selection as channel” button at the bottom (looks like a little dark circle inside a white rectangle). That will make an alpha channel the exact size and shape of your roto’d image. If you’ve got a lot to do, you might want to set up an Action to use in an automated batch process.
But again, AE will read transparency from Photoshop layers. I’m not sure how your project is set up, or what your final goal is, but you might be able to do this faster and simpler with just your PSD file.
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It is easier to destroy than to create.
More fun, too.