Yes, if they flew in earlier and didn’t go to Disney but perhaps rented a car and want to return it and use Magical Express to get to their resort to check in , they can.  Simply input a flight number for that day and rough timeframe when adding Magical Express for them at https://disneysmagicalexpress.com. To find a flight number try this link and pick the right timeframe. https://flightaware.com/live/findflight?origin=bos&destination=mco