The only communication within Azure that goes through SSL/TLS is the database connection with Postgresql Azure service.
No, adding On-Prem environment worker images is not supported. Although, in theory, it should be possible to connect any worker image manually. For that, you would need to start a new worker image and configure it with the Redis connection parameters from the Bitnami deployment. Once the worker is registered in the Redis broker, it should be listed in the Airflow UI.