It is unfortunate that this is not the case. Ai-chan too had entertained the thought of inviting other authors to write stories inside a shared universe. It's too bad that a series is registered to the author instead of floating with the author credited as the creator. But Ai-chan also understands that Scribblehub wants to respect the author's copyright and making the series float would be open for copyright abuse.
Ai-chan doesn't have any suggestion on how to get this to work, but Ai-chan would like to have this feature implemented.