If you have some on order and are expecting it anyday, I would wait and be safe. You don't need to add it at the very beginning, just as soon as you can.
will the fermentation or anything mess up the ascorbic acid in it. I understand what youre saying too. Its better to know what you haave from the get go. Is there a way to measure the ascorbic acid?
If you have started fermentation it will not go bad without the ascorbic acid. Since it was already added you probably don't even need it. You could add it at a half rate of directions on the bag/bottle and have enough to keep it from browning.