Hi,
Currently, the Kafka Connector for start events is based on a NoneStart event. You can see this in the corresponding element template:
[...]
"appliesTo": [
"bpmn:StartEvent"
],
[...]
Since NoneStartEvents cannot be used to initiate an event subprocess, you cannot use the Kafka Connector.
However, the Kafka Connector for intermediate events can handle message events. It may be possible to adapt the corresponding element template to work with message start events of event subprocesses. But, be careful: The element template cannot distinguish between message start events of event subprocesses and normal processes, yet it will only work for the former.