If you want to use Transponding for bi-directional layout communication and are using Digitrax LocoNet, you will not have to add any "feedback bus" since this is already incorporated into LocoNet's design!
If you want to use Transponding with a non-LocoNet system, you will need to add LocoNet. Non-Digitrax control systems require a separate feedback bus for the layout to achieve more limited types of bi-directional communication anyway so, you can simply add LocoNet instead of that feed back bus.