I also would love to know if this feature is in the works, or if there is a user friendly work around.
What we have been doing, and most of the time it works ok, is we open up in an editor the 2 json files we are working with. Then we copy and paste just the skin information from one file into the other.
Then we use the import data to basically create a new spine file based on the edited JSON.
It works ok, but is a tad bit tedious.