The whole thing (league and cup) is based on predictions made for the average home gate before the season started. I don't think late entrants will be allowed I'm afraid.
Don't see the point in the cup one. Given that the 1st round was the Leeds match it would obviously favour those having high predictions - but the prediction was for an average for the season! Not that I'm bitter at a 1st round exit at all!
Also, the second round is against West Brom who undoubtedly bring a fair few. I think that kind of is the point - to keep a bit of interest going for those whose predictions are going to be way out in terms of league success.