1. Click on the bw layer, go to the channels (window -> channels if it's not open), ctrl click on one of the channels (this will select the white parts of the image). Then click on the layer where you want to add the mask. Last step is to click on the mask icon in the layers panel.
2. #1 Almost the same as above: ctrl click the existing mask, select the layer where you want to add the mask, click the mask icon. #2 You can also alt drag the mask from one layer to another
PS: There might be a more efficient way to do the first one, this is the only one I know
