Import modified callActivity into modeler

I am a new flowable user.
I created a BPMN using flowable modeler, this BPMN contains a < callActivity>
Then i manually add < extensionElements> inside this < callActivity>, like this :

< callActivity id=“call-process-aaa” name=“Process aaa”
< extensionElements>
<activiti:in source=“identity” target=“identity”/>
< /extensionElements>
< /callActivity>

The issue is : it is impossible to import this modified BPMN into the modeler. Do you know why ?

Only when the extension elements match exactly what the Modeler would export, it will import correctly.
Have you tried using the flowable: namespace?

Thank for your answer
i do not undertstand the meaning of : “using the flowable: namespace”