this sounds like a job for exmerge it will allow you to take an exact copy of the mailbox and export it to a pst file.
http://www.microsoft.com/downloads/details.aspx?familyid=429163ec-dcdf-47dc-96da-1c12d67327d5&displaylang=enYoull want to run this as the two step method where you will export the PST. The one step method will try to export the email and then import into an exchange mailbox you dont want that. Pretty simple tool to use and shoudl do what you need.
