Stefano,
There is no tool built into the NoSQL module to export the data to an SQL database. Depending on your level of technical skill you could extract the data from the REST api and then import it into an SQL database using many 3rd party tools.
If we are talking about converting data from a Mango instance running on NoSQL to a Mango instance running on MySQL:
You could try to export and import the data using export and import as CSV functionality. You could publish all the data to a new instance running on MySQL using the Persistent TCP module There would also be ways to write Meta Data source scripts and Scripting data source scripts to do this, this again is quite technical so I won't go into it here.