I was wondering is it possible to have a temperature coming from a KNX device to also send to the mqtt network? So that the openhab could act as an gateway between mqtt and KNX? Or any other things supported by Openhab like Z-wave or something
Yes. This is a core capability of OH.
Simply just assign a MQTT,KNX and z-wave address to an item - then that’s it?
For the most part. Depending on how the three interact (e.g. should a value received from KNX cause that value to be sent to z-wave?) you might need to use autoupdate=“false” which will prevent the value from being forwarded to the other bindings.
With this complex of a setup though I would probably use a Rule and a proxy Item. The proxy Item is not bound to anything and is what your other rules use and what you put on your sitemap.
Then create a separate Item for the MQTT, KNX, and Z-wave Items.
Then create rules which trigger on commands received by each of these four Items and forward the command/message to the other three as appropriate. For a fake example, if you receive a command on KNX, send it to the MQTT and Proxy but not the Z-wave. We are just talking theoretically right now so I can’t tell you any more specific than that.