Load Outlook
Click on File
Select Account Settings→Account Settings…
Your email account will be listed, double click on this
In the next window that pops up select More Settings…
Click on the Advanced tab.
Click on the Add… button
Enter the username
in the Add mailbox field
Click OK
Click Apply, Click OK
Click Next, click Finish
Click Close
This new mailbox will show up as a new Mail folder under your existing mailbox.