Thanks toni.sm,
again you are very helpful. I checked your ROS extension code in GitHub again and was able to create a new ROS prim type. I can also publish to odom/tf in a way similar to RosDifferentialBase.
My main problem currently is that I am not able to retrieve the time the RosClock uses in order to set the correct timestamp in my messages (See my other new topic: How to get ros time within extension)