You can, but remember you're then stuck with paying the "gate" price for the pass.
If you can buy before your trip, and thus pay the "Pre-purchase" rates for the entire stay, you can often get the day you were wanting to drop for a fraction of the cost.
For example, the advance purchase price for an adult 7-day UPH is $347. However the gate price of the 6-day UPH (dropping arrival or departure day) is $335. So, that additional day is really only costing you $12.
Food for thought...