So, this is a great story, I think!
Yesterday, I received a message from Joost about converting json to xml on CPI. Because of the time difference, I did not answer him before today. I asked him to elaborate his question, and he replied that he had bought Figaf´s SAP CPI course and found all the answers.
A message like this makes me very happy, and it confirms that Figaf´s SAP CPI course is very easy to use.