OS X Mail - How can I setup multiple e-mail addresses under the same mail account?
To add another email address to an account you add it to the email field. 

Each address is separated by a comma.  "," 

But note just because you can add it does not mean you email provider will accept an address from a different provider/domain.



Edit: As of macOS Sierra, the procedure is slightly different. The "Email Address" field is now a dropdown, and you add additional addresses by selecting "Edit Email Addresses..." from this dropdown.