I recommend against having Outlook users connected to a .pst on a network share -- it is not supported by Microsoft. Auto-archiving in Outlook requires a local .pst file on each user's machine.
It sounds to me like you're looking for a proper archiving solution. I'd recommend reviewing third-party archiving providers, or evaluating the native archiving in Exchange Server 2010 when used with Outlook 2010.
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This was first published in April 2010