1. Load Outlook
2. Click on File
3. Select Account Settings→Account Settings…
4. Your email account will be listed, double click on this
5. In the next window that pops up select More Settings…
6. Click on the Advanced tab.
7. Click on the Add… button
8. Enter the username in the Add mailbox field
9. Click OK
10. Click Apply, Click OK
11. Click Next, click Finish
12. Click Close

This new mailbox will show up as a new Mail folder under your existing mailbox.