Hi Fernando,
your approach to convert JSON towards XML is OK, please check the content help for detailed usage of this atom (seems to be greyed out in your screenshot above).
In an actual Framework version please open Help -> Online Help and search for "json" and the following document "Conversion XML to JSON" will be displayed.
The following screenshot with an example is taken from this document:
Best regards
Bastian
P.S.: Which Framework version do you use at the moment (Maintenance -> System Info)?