To create add-ons, all you need to do is create a mod with an event called 'hometowns_events' and assign characters by historical ID a barony to be born in the same way as this or existing addons. Then if you have Hometowns running, it will read and run the event on game start automatically (in place of the original Hometowns one which adds birthplaces for vanilla starting characters).

The entry in the event should look something like this: