Add a computed value to a device making Home Assistant aware of new sensor

[dirstel]

I've added a user-extension which calculates the dewpoint for any device message containing temperature and rel. humidity, so this has not to be done elsewhere and values may be logged directly to an influxdb.

As I'm using Home Assistant too, I'd like to add the calculated value to the device so it will be exposed to Home Assistant as a separate sensor. This is, where I'm stuck:

• How to change the device definition to allow the dewpoint to be reported as a sensor?
• Is there a way to configure this?
• Do I need to call some API in order to add this?