View Window - Edit | Create Alpha Channel...                    Shift+H



Use this menu selection to create an alpha channel in the open image.



Feather. Select the number of pixels for a feathered border around the selection.


Combine with existing alpha channel. Check this option to combine the new alpha channel with any existing alpha channel data.


Mask non-selected area (inverted). Check this option to exclude the non selected area instead of the selected area.