I need several copies of a specific bone in the same state as the original bone. If I change the original bone, I want these changes to be in the copy as well. I found very annoying to synchronize states of the copies manually and decided to automate it. Then I tried "Import into an existing skeleton" option and you know what happened next.
You say that this option is only for photoshop. I just imported the modified JSON into a new skeleton/project and everything works as it should. Ok.