- Create a second account in the preferences dialog.
- Use the same email address, full name, host name, user name,
- Set the password field to something incorrect (like "1"). You can't leave this field blank
- Set the SMTP host to some mailserver at work you know you can relay through
- In the accounts options, enable the account but uncheck "Include this account when checking mail".
It's not a great solution but it gets the job done.
Note: If you give the two accounts different "email address" lines, you will get a selection box when you are editing the email to tell Mail.app which account to send the email from. In this case, however, I didn't like that so I purposefully made the email addresses on the two accounts the same.

