Currently it is not possible to add extension elements.
Sorry I forgot about this ticket as I fixed it. See commit https://github.com/camunda/camunda-bpmn-model/commit/e6f7ec300b44ef997ca57851f7614942704247ea for the fix.
Sorry I forgot about this ticket as I fixed it. See commit
https://github.com/camunda/camunda-bpmn-model/commit/e6f7ec300b44ef997ca57851f7614942704247ea
for the fix.