1.    Open Outlook and goto Account Settings.
2.    Select Data File Management.
3.    Goto the Data Files Tab.
4.    Click on Add.
5.    In the New Data File Window; select Outlook Personal Folders File(.pst) as type.
6.    Click OK.
7.    Browse to the location where you have saved your PST and click OK.
8.    Give it a name and click OK.
9.    The file will gets “attached” to Outlook.  This file (mails and folders) will not be at the top but below your existing OST that you use for exchange.

NOTE:   Mails/Folders in this newly attached PST is local in nature and is not available in OWA.  If you want to have the mails and folders of this PST in OWA then you will need to drag and drop them in your OST file which is usually with your Display Name.

Related Links:

Outlook Web App

Hosted BlackBerry

Hosted BES

Dedicated Exchange Server

Managed Exchange Server