Both XML and JSON are just specially formatted text files with different syntax. It looks users want's to frequently convert one to each other and thus some programs, like the oXygen XML Editor provide this functionality to its product. There is a dedicated XML to JSON conversion function in oXygen that might help you, as it is likely it can also perform the json to xml conversion.
You might also be interested in one of the online JSON converters, for example Code Beauty or JSON Formatter that can be used to convert, format, beautify, mimic or compact json files.