Hello John Sheehan 9457,
Yes, you should be able to use Ethernet-over-power adapters to connect your Google WiFi secondary units back to the Google WiFi primary. Just connect one of the adapters to the LAN port on the Google WiFi primary to your powerline network is on the "inner" network created by Google WiFi. Then, any other powerline adapters should join that network and Google WiFi secondary units can be connected to them. Here's a help page that describes this from a pure Ethernet perspective: https://support.google.com/wifi/answer/7215624?hl=en
Do be aware that powerline networks are highly variable in terms of reliability and performance. They may work fine for you, but they may also provide different performance depending on which phase of your home's power each one is connected to (I'm assuming something like a standard 220/110 breaker box where half the 110V circuits are on each of the two phases). So, build the system up one piece at a time, and test the performance using something like a laptop connected via Ethernet to get some performance measurements from just the powerline network before adding more to it. If you find the powerline systems don't perform well enough for your requirements, you may consider MoCa adapters (if you have coaxial cabling in place already) or just running Ethernet directly (which will perform the best).
Google recommends a maximum of five access points in a single system. It's possible wiring them could allow a slightly higher number, but that's not a guarantee.
