1. Create new document 2. Right-click on a layer 3. "Convert" -> "to Transparency Mask" 4. Copy and paste the resulting transparency mask The mask is now a regular layer with the color space "Alpha" It can also be reproduced using a filter mask, and presumably other types of masks.
Hm, I cannot get this to work. It always pastes a mask for me. Could you check again? Maybe there's something missing in the description?
If I missed a step, I wonder what it was... I'll mark it WORKSFORME and reopen if it happens again.