Hi Steven,
I'm not an Exchange expert but as far as I know, PST is a local file reside in individual client's machine but not on the mail server..
But you may take a look at the Exmerge tool: http://support.microsoft.com/kb/265441
You can use Exmerge to migrate users between different organizations and sites by copying the users' mail to .pst files that you can then import into the new organization or site.
Regards,
Colt