Currently, we don't have this tool out of the box. Also, no part of our code writes json.
You wouldn't need to store it as json. It could just as well be a Unity ScriptableObjects if you want to go that route. A Skin is primarily just a dictionary of skin entries. If you read the Skin class, it's pretty straightforward. Just need to rewire it at loadtime since it does store Attachment objects which don't exist until after the Skeleton data has been loaded.
If you wanted to serialize attachments too, that is likely also doable via Unity ScriptableObjects.
You're probably already aware of this if you were doing custom skins. But for the benefit of other users, here's a link for more on how it works: Spine-Unity Mix and Match