I don't know how it's implemented, but I see no reason why adding/removing emails to/from an existing account (while logged in) wouldn't be trivial to implement.
The only thing you need to be careful with is to send a notification to the email being removed, send notification to all emails when a new one is added, and enforce some kind of expiry period before a removed email becomes inactive/added email becomes active.
The only thing you need to be careful with is to send a notification to the email being removed, send notification to all emails when a new one is added, and enforce some kind of expiry period before a removed email becomes inactive/added email becomes active.