Photoshop is a wonderful tool but, unless you use it everyday, you will forget some of the secret keys needed to make it work for you.
Edit a Layer Mask
ALT+Click on the layer mask to enter edit mode. From here you can paste in data from another layer.
Generate Image Assets: Include Alpha/Transparent Border
To include a transparent border when generating assets, add a layer mask to the top Group/Layer with the bordered area you want to included in the generated images.