I'm planning to sometime do a box set with the first three books in one series and include the novella that currently is only available as a reader magnet. It's not a prequel or a sequel (there's been some talk lately of authors writing long epilogues that they make available only to people who sign up for their newsletters). The novella's action spans books 2 and 3 of the series. But even saying this, I realize that I'll need a new reader magnet before doing the box set. Whoops. To the back burner. So, yes, people do add previously unavailable or unwritten stories to box sets to sweeten the deal. In my case, the series is in KU and the box set would be, too, which is why I'd want to withdraw it from being only a reader magnet.