You are in luck. There is a tool designed for those in your position. It is a .pst (Outlook Data File) importer. You can find it in the downloads section of your Admin console, which, if installed to the default port, can be found at
https://yourservername.com:7071 (don't forget the http
s!). Anyway, you can also download it directly (assuming you are using 5.0.2) at:
https://yourservername.com:7071/zimb....2_GA_1967.exe
Ask if you have any questions, or (even better) see the documentation
here. It's pretty self-explanatory, though. Good luck!