I checked the Help topic (shown below) but it only mentioned how to add a story of your own to a series. Another option to add a story to a series seems to be choosing the option Add a Story to Series under the Manage button. A pop-up window then asks for the story title. This is the way I added Cupid Central by @aditus to my Viking Valentine series. I wanted to add the new Anthology story A Hell of a Mess as well, but when I typed the story title, the system couldn't find it. In fact, the Story Title pop-up editor would only find stories by me, and not anything by other authors. However, since I'm co-author of A Hell of a Mess, this is still a bit strange. On the other hand, I don't get the option to use the green + button described in the Help topic, so I guess the system is consistent.
Do you have a suggestion how to solve this, other than asking a GA admin to add the story ?