How do I add more stories to a collection that is already written?

On the book's page, press "Add stories" above the chapter list (from version 1.9.20; in a book that is not a collection the same button is "Extend with AI"). Choose how many stories to add and, if you like, say what they should be about, for example "one story set at sea, one about a wedding gone wrong". BookyAI plans the new stories against the ones the book already has, so each new story has new people, a new place and a new turn, and writes them after the last story. They appear in the list as they are written, and you can edit or regenerate each one afterwards.