"We didn't specifically commit to adding D/FW," Kelly said, "but that certainly is in the spirit of what we agreed to do."
Overall, the partnership provides about $50 million a year in annual revenue, he said.
Cox said the ATA deal could be a first step toward bringing full Southwest service to D/FW.
"We are hopeful that over time, Southwest will see the wisdom and benefit of having the access to D/FW," he said.
And Wagner of American Airlines said it demonstrates that Southwest could choose to compete from the larger airport.
"This illustrates the fact that we do compete with low-cost carriers at D/FW, and there's more than enough room for competition here," he said.
But Kelly said the airline will remain at Love. "We still don't see any point to splitting our operation between the two airports," he said.
The service from D/FW becomes available for sale Jan. 3, and the flights will begin Jan. 11.
Stock in Southwest and American jumped Thursday amid optimism that the airline industry will recover next year.
Shares of Southwest (ticker: LUV) closed at $16.61, up 35 cents or about 2 percent. Shares of AMR Corp. ( AMR), American's parent company, jumped $1.83 per share, or 9 percent, to close at $21.44 per share.