If both had an interim "format free" state then yes, you are right, it should be possible.
jFiles does have such a state, but alas xFiles does not.
So it would be relatively easy to create xml from Json - simply by adding an output method to jFiles so save the interim state to xml instead of json.
In practice everyone would want waaay too much control over the xml - this should be an attribute, that should be a field, and so on.
I'm sure there are tools to do this, but it's not a road I have a need to go down <g>.
cheers
Bruce