I called customer service about this earlier today (and waited a REALLY long time on hold listening to music I didn't like). The CS rep told me he *could* add it, but he was afraid it could possibly delay my order and he didn't want that to happen to me. I was able to order last night via the app before the ship dates got pushed back. Anyway, he seemed nice enough, and I don't think he was just trying to get out of doing it, so word to the wise, for what it's worth. It might be worth waiting to try to add until you get it in your hands.
He asked if I had an Apple store near me, and I said about 40 miles away, 60-90 minutes in traffic, so he told me just to call in next Friday when I get it and they'll add it. I hope that's the case because otherwise I'm going to be driving a lot and standing in a line, exactly what I try to avoid by ordering online.