- You can export them using Google Takeout
- Export them from "Google Maps (My Places)", not from "Bookmarks".
They will be downloaded as a GeoJSON file.
- You can use this site to convert your file to KML GeoJSON
- Once you download the KML file you can import it into your custom map
- This help center article explains how to do it.
Tuesday, August 23, 2016
Google - Convert Google Maps starred places into a My Map
Found this on Google Maps forum: