This is a great thought, and I’ve read a thread somewhere where someone was able to do this. However, I think this is going to depend on your individual setup. For my home, the transformer wires went to the doorbell chime and the doorbell, with another wire coming back from the doorbell to the chime. Thus, everything met at the chime to complete the circuit when the doorbell was pressed (which is what the Nexia device is sensing).
This is where I saw the post about being able to wire to the transformer:
This was from Justin Eltoft’s post about half way down the page:
You do however need to have a 14 volts AC (up to 40 volts AC) transformer. I luckily had the door bell switch 2 conductor wire and the bell two conductor wires both coming to the transformer, so it was easy to find out how to wire mine up at the transformer and avoid an “ugly” extra device mounted up by the doorbell in the hallway. Much improved WAF ( wife acceptance factor).
I ended up using a ST MultiSensor stuck to the doorbell chime. It reliably picks up the vibration when chime makes noise and triggers a acceleration event. Works awesome, is smaller than the other solutions, cheaper, easier installation, and you get an integrated temp sensor to boot. 
If you want to convert the acceleration sensor to a motion sensor see here