To have another mailbox within your outlook first you need to request access from support, once the access has been confirmed you will need to follow the next few steps to add the mailbox to your outlook profile.

1. Click the File tab.
2. Click Account Settings, and then click Account Settings.

3. On the E-mail tab, in the list, click the Exchange account type, then click Change and then click More Settings.
4. On the Advanced tab, under Open these additional mailboxes, click Add, and then enter the mailbox name of the person whose mailbox you want to add to your user profile. If you do not know the mailbox name, ask the person who granted you Delegate Access permissions.