I think I found the “correct” way. So when you already setup the connection between deconz and diyhue, there seems to be no automated updating between the two.
So in order to learn a new device, you do it via the phoscon webui. After the device is properly connected you can start in the hue essentials app with the diyhue as bridge configured a light search. The app says that no device is found, but this lights search seems to trigger a sync between deconz and diyhue.
So after you did the light search in the app, you can see the new device in the list. Hope this helps others. Maybe this hint could also be implemented into the documentation?