Hi! I think if you think the piece stands on its own, you're welcome to add it to the collection. If you want to add more to the story, I would probably still recommend making it a series and adding the sequel material as the next story in the series, just to avoid giving the impression that the original story was, like, a 1/? (or however many chapters) sort of story.

But I don't see an issue with hinting in a note that you might add more to it later, as long as you think it's a satisfying read on its own!