Yes, you can take Magical Express. Arrival on a plane is not required.
Simply call the DME people directly (866-599-0951) and tell them what you want to do. They'll ask you about what time you're planning on taking the bus and get you booked in the system.
The only aspect of the arrival process you won't be able to use is the yellow tag luggage delivery. Your bags will go on the bus with you to your hotel.
-Rob