The SkyTrain is a monorail that runs from the airport to the convention center and rental car agencies. No hotels are connected to it but I do not see any reason why you could not walk from the Convention Center station to the hotel, assuming that you are allowed to get off even when the convention center is closed. This is a new service to Atlanta, so I'm not sure what its hours of operation would be when the conventions center is not open.
You should be able to use your hotel's free airport shuttle to get to and from the airport an the MARTA station. You would not be required to show a boarding pass to get to the area of the airport where the MARTA station is located, and it's possible that your hotel shuttle would take you to the College Park MARTA station anyway.
Edited: 4:02 pm, April 07, 2010