Saulgoode i am wrong or you wrote (maybe years ago for a previous gimp version) a sort of "paste as layermask" script (name was different but that was the main function )
I probably did, way back when. Rather than revisit that, here is a script
which might simplify your task a little while being a bit more general in its utility.
It is largely based upon the GAP's "Copy layer mask from layer above" function in the Frames Modify command.
Here is the description:
;; This script adds a layermask to the active layer based upon
;; the layermask of the visible layer above it in the layer stack.
;; If the active layer already has a mask, it is replaced.
;; If the above layer has a mask, that mask is added to the active layer.
;; If the above layer does not have a mask, a grayscale copy of the
;; above layer is added as a mask to the active layer.
;; If there is no visible layer above the active one, a mask is added
;; initialized from the selection (avoiding the dialog popup).
The command, "Copy Layer Mask From Above", is added to both the "Layer->Mask" menu and the Layers context menu (that appears when you right-click on a layer).
With this script installed, you could dragon drop your mask layer from the other image onto the target image, placed above the layer to which it is to be added as a mask. You will have to remove the layer (or hide it) after running the script so it doesn't save that many steps, but I think it is generally an approach that should prove useful in other situations (I have used a similar script for a few years now -- for some reason I find copy-n-paste to be a bit tedious).