How can I create one sub menu for startEvent?

[tcguoxing]

I want to create one event type based on startEvent which its structure like below:

<startEvent id="sid-409523CE-5537-4988-90FA-59938F90E8C5">

      <extensionElements>

        <flowable:eventType xmlns:flowable="http://flowable.org/bpmn"><![CDATA[lll]]></flowable:eventType>

      </extensionElements>

    </startEvent>

I want to add the extensionElements using sub-menu(or another professional name) like below:

But I have no idea how to add Register Start Event in startEvent Settings, how can I do this?
My modeler is bpmn-modeler.

[nikku]

Probably answered by Can We Add Custom Element And Shape For Drawing - #2 by nikku.