Unfortunately, once a Story Album has been created, you can't add any other picture. Using the menu (
) option, you are allowed only to edit title, change theme or cover photo, view the album as a slideshow, share it, export it or print it.
If you want to add pictures to a new Story Album, launch the utility press the Create album option and select From Gallery or By tag information. Both options are explained on the official web page of the program.