Apply selection to layer mask

And if you want to do it in a "non-destructive" (reversible) way - put the current layer with the existing mask in a Group, ctrl+G, and put a new mask on the group.

Or ditto smart object, but Group is usually simpler.

I wonder if using Alt + Backspace for fill selection with foreground color is the solution you are looking for?  I use it often.

From there, you should be able to output a luminosity mask as a selection and then fill the selection using the above method.