As I understand it, the one-shot is a single chapter in the multi-chapter Scooby Gang Time story. If that’s the case, I’m going to say that the site rules wouldn’t prohibit taking that single chapter and using it to start the spinoff story. I would just ask that you stick a little note somewhere, even as a footnote author’s note to the chapter, that this was originally a one-shot in Scooby Gang Time, and it took on a life of its own. You can add a link to the original story as well.