This is not a fantastic solution — and it does not use the API — but it might fit your needs. Instead of using the API you could write a browser console script that adds each new item. You could probably even run this through a headless browser and remove any manual interaction altogether.

eg.

fetch('https://example.com/newLocations')
  .then((response) => response.json())
  .then((newLocations) => {
    newLocations.forEach(location => {
      document.querySelector("#mapsprosearch-field").value = location.name + ", " + location.city; 
      document.querySelector("#mapsprosearch-button > div").click();
      document.querySelector("#addtomap-button").click();
      // etc.
    });
  });

—

UPDATE

Re-opened as of Feb 2021 — API support for this has been assigned. So hopefully we get support soon! https://issuetracker.google.com/issues/35820262