If I understand you properly, you are wondering how you can add threads to a story that has not yet been approved and posted on CHYOO.
I don't know that it can be done - nor, for that matter, do I think you'd want to. Based on how long it takes for a story to get approved in the first place, someone must be reading them and approving them by hand, rather than a computer doing it. Therefore, if you go adding threads to a story that hasn't been approved yet, four things might happen, and three of them are bad:
1: The story might get approved with the threads. This is the good option.
2: The story might take longer to get approved because it's a loger story and the guy doing the approving must take longer to read it.
3: The story might be approved, but the addition threads lost and you'd have to type them again.
4: The story might not be approved, and you'd have done extra work for nothing.
So I'd suggest you write your additional threads and keep them on your computer. Then, when the story is approved, just go in and add them all. I did this with Sappho's Amulet and it worked well.