Hello, if you want to add new skin to the document and you don't want your existing images to be changed in the other skins, choose Ignore instead of Replace.
It is also important that the new skin doesn't have a name that matches other skins, or it will be detected as being the same skin by the script, which is what I suspect may have happened.
I tested the process of importing new skins in Spine and it worked fine for me on the latest Spine version. If you still encounter problems please send a zip that allows us to reproduce the problem you are experiencing to contact@esotericsoftware.com mentioning this topic.