No, you can't pair your head-unit to the TrainerRoad app.
Head-units and TrainerRoad are both receiving devices that you pair your cycling devices to during training. TrainerRoad acts as your head-unit during your indoor workouts.
Pairing devices to both TrainerRoad and your head-unit
If you're connecting over an ANT+ connection, you can pair your cycling equipment to both TrainerRoad and your head unit/fitness watch at the same time.
Note: If you're pairing via a Bluetooth connection, you'll need to only connect your cycling equipment to TrainerRoad. Bluetooth equipment can only pair to a single receiving device at a time.
Check out this section of our Help Center for more information on connecting devices.