Tricky. Do you need this for a specific page/asset publisher?

In any case: You probably would need to add a listener for groups to notice when a site is added.

https://help.liferay.com/hc/en-us/articles/360018170891-Model-Listener

And then you would need to read the configuration for the relevant asset publishers and add the id of the group to the xml. But this is quite difficult since you won't find a lot of sample code for something like that.

Well, I can only guess what has to be done. I think, you have to iterate through all pages.

 List<Layout> pageLayouts = layoutLocalService.getLayouts(-1,-1);

// That's probably too many, you probably want to fetch only layouts of a particular site

for(Layout pageLayout : pageLayouts {

// Now, you need to find the portlets on these pages
  LayoutTypePortlet layoutTypePortlet = (LayoutTypePortlet) pageLayout.getLayoutType();
  List<String> portletIds = layoutTypePortlet.getPortletIds();
  for (String portletId : portletIds) {

// Now, we need the portlet preferences for each of these portlets
    PortletPreferencesIds portletPreferencesIds = PortletPreferencesFactoryUtil.getPortletPreferencesIds(pageLayout.getGroupId(), pageLayout.getUserId(), pageLayout, portletId);
    PortletPreferences portletPreferences = portletPreferencesLocalService.getPreferences(portletPreferencesIds);
     String values = portletPreferences.getPreferences();

// Check what's in there, then update it
  }

I would try that on one page, to check if it even contains the relevant information. I could be totally off here, I just pieced it together from the API.