Access CMMNDI Classes

i created some CMMN programmatically using the Camunda Model api. The resulting XML does not have a cmmndi section and thus is not viewable by either the camunda modeller or Can someone explain to me which classes to use to control the rendering/positioning of CMMN diagram elements? I found the corresponding classes for BPMN, but not for CMMN or is it recommended to use XML directly?.
Any help would be appreciated.