There is no serialization back to json from a modified skeleton in the Spine-Unity runtime.
In general, it is not considered good software architecture to put application-logic such as persisting application state into an animation/graphics module.
The recommended way would be to store any character setup state in the same way you would store the character's name. In case of multiple equippable items this could be a list of item-IDs.
Then during startup, you would again equip all things one by one - setting attachments, skins and performing other non-graphical application logic such as modifying player attributes (e.g. armour value). This way you can be sure to have clean consistent state across graphics, gameplay, etc.