I've got the same problem, so short stories small enough to write in one day just don't happen for me. The entry I'll be submitting today will almost certainly be just the first one third of what I have in my head for the story, because I just can't write that much in one go. Looking through the calendar, I've realized that the final portion will be a good fit for Round Four.

I plan to submit 'part one' today, and the other part three weeks from now, as separate static items. I'll combine them after NaNoWriMo is over.

Incidentally, it won't bother me if the judges for these rounds don't think doing this fits the contest requirements. I'm doing this for my novel, not to win a contest.
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Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown