It is possible to connect to another users mailbox (if you have proper permissions) using OWA. The trick is to append the users mailbox to the OWA URL. For example, if the URL to your OWA was: https://webmail.mycompany.com/owa you would simple go to https://webmail.mycompany.com/owa/[email protected]. You can then log in with any user account that has the proper permissions.
Found this information at TechNet.