It manually done with Gimp. There is no tool. The atlas.dds is a just one file that consists of all the separate images.
Unpacking and packing is strictly forbidden if you want to preserve the quality of the buttons/images! However, you can cut out the images and copy them into a new separate file.
The atlas.dds images are a relic of the beginning. The former idea was to have just one file for all buttons. However, its impractical sometimes if you want to change something.