You might be able to connect the you Sena to the BT in the car if it's a newer model with hands free. The problems with this as I see them are:
1. Will they connect? Easy test in the driveway. Each Bluetooth device connects to the Cell Network independently.
2. How to activate communication from car to Sena. Check in driveway. The phone itself becomes a new communication device? May or may not be hands-free bluetoothed to the car.
3. Distance car can receive from Sena. A walk down the block test if they connected and communicated. It's no longer dependent on Bluetooth once connected to the Cell Network through the app.
4. Distance Sena can receive from car, bet this will be an issue, but a simple enough test with a walk down the block. See also above... their video shows riders on different continents being able to communicate.
Think JMartin has a better idea to test.