Found this in my research. Basically two parts to this little device:

Part 1: takes an audio signal from a 3.5mm line in jack and converts it to a PDM waveform as required by Google Home

Part 2: intercepts the speaker wires, cuts the voltage back, and directs it to a 3.5mm aux jack.

Kit: SnekTek Google HOME Line IN&OUT external microphone kit

I have not tried this myself but thought it was too cool not to share. Will share again if I end up getting it... it's a lot of time and money for a house full of Google Homes...