I just found this java tool:
https://github.com/thomas-villagers/gdxtococos2dhttps://github.com/thomas-villagers/gdxtococos2d
update: previous link doesn't work anymore, I found a fork:
https://github.com/midworld/gdxtococos2d
run on terminal:
java -jar gdxtococos2d-0.0.1-standalone.jar <inputfile>.atlas <outputfile>.plist
...and done!
it worked for my game.