Add Additional Mailboxes in Outlook 2010
STEP 1: With Outlook 2010 open, click the File tab.
STEP 2: Click the Account Settings button, then click Account Settings again.
STEP 3: Click your email account, then click Change.
STEP 4: Click More Settings.
STEP 5: Click the Advanced tab. To add an additional mailbox, click Add.
STEP 6: Enter the Alias of the mailbox or the Display Name; then click OK.
For example, if the display name in the GAL is Doe, John then enter that in the box. If the e-mail address of the mailbox is firstname.lastname@example.org, enter john.doe in the box.
STEP 7: Click OK.
STEP 8: Click Next.
STEP 9: Click Finish.
STEP 10: You should now see your additional mailbox within Outlook.