In Outlook go to File > Account Settings > Account Settings
Under Email select your email address and click Change…
Click More Settings . . .
Click the Advanced tab > click Add . . .
Enter the name of the mailbox you want to add and click OK (if more than one come up, select the one you want) > click OK
Click Apply > OK > Next > Finish > Close
This mailbox should now show up under your mailbox in outlook.