Microsoft developed a PST Capture tool that will help administrators search the network and discover pst files - then import them directly into users' primary mailboxes or archives.
Installation instructions are available at Install PST Capture
There are three parts to the capture tool (which requires Outlook 2010 64 bit):
PST Capture Central Service: The Central Service maintains the list of all PST files found in the organization and manages the data as it’s moved to the Exchange servers or Exchange Online.
PST Capture Agent Discovery: PST Capture agents hat are installed on computers in your organization. The agents also send the PST files they find to the host computer when an import operation is started on the PST Capture Console.
PST Capture Console: Use this interface to configure PST searches, specify the target mailboxes for PST files, and track the status of PST import operations and reports. The console can be used to import PST files stored on network storage devices where you can’t install PST agents.