The format for these files is as follows:
8 bytes: identifier, should be 01 00 00 00 00 00 00
4 bytes: (ulong) number of items in the atlas
then for each item:
512 bytes: zero-padded wide string containing category
512 bytes: zero-padded wide string containing file name of the texture if it had not been put in the atlas, the game uses this file name to look for the texture.
8 bytes: (two floats) top left corner in texture units of the part the atlas texture that is for this texture
8 bytes: (two floats) width and height in texture units
8 bytes: (two floats) width and height in pixels.
What an example may look like (this is the first part of france_diffuse.atlas and the associated .dds from the NTW demo), see also the attachment for a visual explanation.
If you want to copy and paste from one atlas to another you would copy from the name of the category up to (but not including) the name of the next item's category. Be sure to also fix the coordinates, otherwise you'd get some random part of another texture.