Custom mask from image

Hi,
I have scanned an image with film grain which I'd like to apply as a mask in Capture One.
I've noticed masks are saved in .comask files; I've found no way to import/export them to a monochrome image format.

I know i could export the image and apply the grain in PS; I'd prefer however a simpler and quicker approach where i don't have to leave C1.

Suggestions?

Mat