Hello toanpremium
When you pack textures with Spine, you get an atlas text file and at least one PNG containing all the images of your exported skeleton.
If your question is "how can I get one atlas text file and each skin in its own separate PNG," Spinebot's answer is correct. You just need to organize your skin images into folders and choose the appropriate settings explained in the guide.
If your question is "how can I get a dedicated atlas text file and PNGs for each skin", the only way for now is to export multiple times, leaving the export checkbox active for one skin at a time.
However, we have just merged support for this feature into our 4.3-beta branch (see the dedicated issue here). This means it will be available in the next beta version, which should be released soon.
You can subscribe to that issue to stay updated on the release of this feature.