getting stored mail out of outlook pps to another format is a pain in the arse, as i recently found out when migrating 20,000 archived messages from a winxp outlook 2003 setup to a macbook!
in the end i've gone with using thunderbird for mac rather than the osx built in mail program. nearly every method i found of converting did horrible things like remove email attachments, remove email sent/received dates, or simply created thousands of blank emails!
i found the only reliable way of doing this was to connect to an imap server on the outlook box (i had always used my blueyonder email as pop3 but they support imap too), and move over the messages from my outlook local file to a folder i created on the imap server (i did this about 1500 - 2000 messages at-a-time which took probably 10-15 mins each go), then connect to the imap server on the mac and download them all again on there before moving them to local storage. it was time consuming, but at least everything is intact and stored properly in a nice universal format!
if you can get access to a free imap server i'd recommend this method, everything else i tried had drawbacks!