Adding an Additional Mailbox in Outlook 2013 is very similar to the process for Outlook 2010 or Outlook 2007.
Open Outlook 2013 and Click File.
Click Info, Account Settings, Account Settings…
Highlight your current email account and click Change
Server settings box should appear, click More Settings.
Click the Advanced Tab and Click Add
Type in the name of the additional / second mailbox you want to add and click OK.
Confirm the new mailbox was resolved by the server and Click OK to save.
You will be back on the Server Settings screen. Click Next.
Click Finish
Click Close on the Accounts Settings page.
Your new additional mailbox should be listed under your main account.