Parece que isso é descrito no StackExchange da Web Applications em esta resposta , citada:
Yes. Add it the same way you'd add any other email to send from:
- Click on Settings in the upper right.
- Click on Accounts and Import.
- In the "Send mail as" section click "Add another email address you own".
- Add the "plus email" and finish the setup.
- Compose an email and select the custom "plus email" in the From dropdown.