Feeds Node processor mapping

I want to import XML data from a remote server. The Feeds module has to interact with an API that outputs the data in a XML format where the actual content is in JSON format. Here's an example of the XML file:

<serverResponse>
  <status statusCode="0">OK</status>
    <messages>
      <user/>
      <tech/>
      <alert header="_*Alert*_"/>
    </messages>
  <content>
    {"appointmentSet":[{"appts":[]}],"bookingAllowed":false,"maxBookAheadPeriod":25}
  </content>
</serverResponse>

What I try to achieve is to let the Feeds module grab the <content> data and put it in a node text field. Then I want to use the drupal_json_decode function to decode the JSON data in readable text.

In my XML Xpath parser settings I've used //content for the Context value. But I've no idea what to do with the processor mapping. Can someone please help me? Does the Node processor mapping require a unique identifier for the GUID or is this not mandatory?