This information is a couple of years old, but should still work...
Convert Windows Outlook mail to Mail.app
Thu, Oct 3 '02 at 09:13AM from: stephen.bates Converting the corporate Windows user who has all their old email in .pst files for years back? Help them on their way by getting their mail out of the proprietary .pst format and into standards based mbox format. The process sucks a little bit, but it does work if you have both machines on the same network.
On your current Windows machine:
1. Start on a Windows box with Outlook and .pst files (Personal Folders)
2. Download Netscape 4.7X for Windows
3. Launch Netscape, launch Netscape Mail
4. Select File -> Import, choose Outlook 97/98 (regardless if you have Outlook 2000/XP), select Outlook profile (if you have one)
5. Ensure all your .pst files and mboxes are selected (Personal Folders)
6. Wait for the Import to finish, Quit Outlook and Netscape
On your Mac running Jaguar:
1. Launch Mail.app
2. From the Finder, Connect to Server and connect to your Windows Machine
3. Navigate to the Windows machine - C: -> Program Files -> Netscape -> Users -> YourUsername -> Mail -> Outlook Mail.sbd
4. Copy the Personal Folders from the Windows machine to somewhere in your Home Directory (I chose my Documents folder)
5. From Mail.app, choose File -> Import Mailboxes ... select standard mbox files
6. Point towards the directory where you copied the Personal Folders from the Windows machine
7. Ensure all the mailboxes are checked
When it's done, look in the Import Folder in the "On My Mac" folder, and ensure all your imported mail is there. This method should should keep the dates and attachments intact. If you find you have TNEFs, use TNEF's Enough to extract them.