Backup your outlook.pst before beginning. Use an existing IMAP mail account or get a free Hotmail Account. Logon to the account in Outlook for Windows. Drag any messages and/or folders into the IMAP account. Keep in mind they will be moved, not copied.
Now just login to the account from Entourage and drag the folders into the default Inbox. You can delete them off the server when you are finished, or hold on to them for posterity.
If you have a lot of messages this can take some time, but it's worth it if you want to keep your old messages and folders. If you want to restore the messages back into Outlook, just overwrite the outlook.pst with the backup you created.

