Hi Viridion,
Do you mean that you want to set up additional email addresses for the same mailbox?
If yes, you can refer to the following steps to meet the requirement:
1) Log into the admin center with your account.
2) Click the Admin>click Exchange to enter Exchange admin center (EAC).
3) Click recipients on the left navigation.
4) Click mailbox on the right panel and double-click the display name of the mailbox you want to edit.
5) Click email address on the left navigation.
6) Click add icon to add an email address for this account.
7) Select the Email address type you prefer and enter the email address you want to use (here is just an example of smtp email address).
8) Click Save.
If no, could you please give me more detailed information for further assistance?
Thanks,
Anna Shi