No software changes for hello doorbell needed. just a simple piece of hardware and a new product. Simple RF transmitter and receiver system. Wire transmitter to power between power and hello doorbell, and then place receiver on battery powered Nest designed speaker box. simple battery powered RF doorbell system. Simple example in link below.
If You want i can create the nest RF wireless doorbell chime and send it in for production for free. im Losing alot of business on the hello doorbell for my home automation company because there is no wireless chime option.