Can a round be extended if a divisee needs more time?

The short answer is, rounds are automatically extended when the end date arrives but a divisee didn't finish bidding.

An extended round can be closed manually by the administrator when she sees fit to close it or automatically when all divisees finish participating in it.

If all divisees finished participating on the round, even before the end date arrives, the round gets closed automatically. The results of such a round should be respected and the only way to revert its effects is to unassign all its assets, delete the round, recreate the round and have all divisees bid on it again.