Short answer: I think so.
Long explanation: Unless they have made changes to the on-line system, we were able to book ADRs for dates beyond our check out date using the first confirmation number of a split stay last fall. If memory serves this was shortly after the transition into on-line ADRs.
I'd say give it a try...
We have a one-night reservation before the rest of our stay.
Although I hadn't planned on it, I was able to use the one-night reservation number to go ahead and book out for 10 days.