All you need to do is add the new image sizes to the icon
array, so it is as simple as it seems. You do not need to use the '@2x' naming scheme for anything except the Launch images.
Make sure you are running the latest version of AIR, though. You won't be able to submit to the iTunes Store with anything less than 4.0 (AIR is currently on version 13.0) anymore. The icons you are trying to add weren't added to AIR until 3.9, anyway.