Ohh right. Nobody does that! (creating channels off the broker bridge Thing)
The much more useful way to configure MQTT things is to create a number of new generic topic Things, each representing a particular device/box, like a Sonoff or whatever.
Then you configure channels, as many and as varied and you like, belonging to that generic Thing. These can include transformations, link to Items, etc etc.
This has nothing to do with embedded broker.