How to add Google Play support in Anbox?
I'm not really sure about Anbox, but I've made and modified several Android distros, that are unregistered, as you had said. What I had done on these devices was:


Either go to Open GApps and then download the app. Then run it, choose the package and wait for it to download. Use a superuser, to grant GApps permission, and allow it to install all Google apps (as per your package selection).
The second thing you can do is, use flashify, to flash a zip that you can download from Open GApps again.