Actually, you CAN use your Ultimate Rewards points to AirTran.
(1) Transfer your Ultimate Rewards points to Southwest here
http://rewardsfarther.com/rapid_rewards_to_a_plus/
(2) 1,200 Southwest points = 1 AirTran credit. That means 16,000 Ultimate Rewards Points = 1 AirTran roundtrip in coach. 32,000 Ultimate Rewards Points = 1 AirTran roundtrip in business class
(3) Book your AirTran flight using your free ticket. You can also check AirTran on-line for availability BEFORE you take steps one and two above.