Create a black (or any "ink") Solid Color Fill layer with a mask containing an inversion of the scan.
You can open the scan into Photoshop. Cmd/Ctrl-click on its RGB (or Gray if grayscale scan) thumbnail in Channels panel to get a lightness selection then press Shift+Cmd/Ctrl+i to invert the selection. Now add a new Solid Color Fill layer, which will be given a mask automatically. Hide the scan layer.