It’s shocking that I need to steps by step someone today on how to do this. That said I might as well share the info here. *oh and no screenshots*

Open Outlook 2010 and Click File, Info, Account Settings
Highlight the account and Click Change
Click More Settings
Click Advanced Tab then Click Add
Type in the name of additional / secondary mailbox and Click OK
Verify the new mailbox is listed. Click OK to save
Click Next
Click Finish
Click Close