Can not create child elements in the bpmn:diagram tag using javascript (Bpmn-moddle)

I have created a definitions element in xml using Bpmn-moddle. It has two root elements: bpmn:process and bpmn:diagram. Whenever I try to push something in the process tag, it successfully gets inserted. But when I insert something in the diagram tag, it is not being displayed in the xml file. Can someone help me with how to insert child elements in the diagram tag.

<bpmndi:BPMNDiagram id="BPMNDiagram_1">
    <bpmndi:BPMNPlane id="BPMNPlane_1" bpmnElement="Process_1">
      <bpmndi:BPMNEdge id="Flow_1jne4it_di" bpmnElement="Flow_1jne4it">
        <di:waypoint x="-495" y="-380" />
        <di:waypoint x="-465" y="-380" />
      <bpmndi:BPMNEdge  id="Flow_0kpp457_di" bpmnElement="Flow_0kpp457">
        <di:waypoint x="-330" y="-320" />
        <di:waypoint x="-330" y="-300" />
        <di:waypoint x="-270" y="-300" />
        <di:waypoint x="-270" y="-280" />
      <bpmndi:BPMNEdge id="Flow_1irjjnq_di" bpmnElement="Flow_1irjjnq">
        <di:waypoint x="-135" y="-220" />
        <di:waypoint x="-110" y="-220" />
        <di:waypoint x="-110" y="-230" />
        <di:waypoint x="-85" y="-230" />
      <bpmndi:BPMNShape id="Activity_18hvm8y_di" bpmnElement="Activity_18hvm8y">
        <dc:Bounds x="-765" y="-440" width="270" height="120" />