Add as alias or url
https://support.google.com/a/answer/7502379?hl=en
https://support.google.com/a/answer/175747?hl=en
Add aliases to your user if url
Add aliases to your gmail via settings
BEWARE! You will loose access to calendars created with a google account linked to that domain before you thought about the aliases. You will not be able to use your aliases as the creator of the calendar events.