Importing messages from Outlook Express

This isnt the most ideal method but you could set up a message rule in OE that forwards all of this emails back to himself. You would need to turn off the mail receiving within OE by turning off the 'Include this account in Send and Receive all' or just deleting the POP information from the email config.

If you create the rule specifying all mails that contain his email address in the 'To' field to forward back to his address and then apply the rule to the inbox it could work.

You then just setup mail with the correct logins and it should pull them in. Even if there are a lot of mails it shouldnt take too as long as you are using a broadband connection. A lot if ISPs let their users have around 1 or 2 gig for their inbox in total so you might get away with this.

I know all the emails will come as from him self but the subjects would stay the same.

I believe the import options for Mail 2.x are "powered" by AppleScripts that launch the source application & use its AppleScript commands to extract the messages from the app's "native" format in a form Mail's own Applescript commands then insert into Mail. (These scripts can be found in /Library/Scripts/Mail Scripts/Helper Scripts/.)

Because the application itself must do the extraction, you would indeed need Classic support for this to work with OE5 for OS 9, which the Intel Macs do not offer.

As a workaround, you could temporarily install OE 5 on your PB G4, copy your dad's OE files to it, & import its messages to the PB's Mail, assuming you have Classic support installed on the PB. (To keep things simple, you could create a temporary OS X user account for your dad on your PB, so that your messages & his do not get mixed together.) Once imported into the PB's Mail app, you could then copy the temporary user's ~/Library/Mail folder to your dad's MacBook at some location other than his own ~/Library/Mail/ folder (so it won't replace any messages he already has stored in Mail), & he can import the messages from that folder's mailboxes.

hmm i actually need to do the same thing.
perhaps thunderbird is able to import outlook express mail?
and then you could use mail to import the thunderbird mail?

Perhaps google can help track this down.
I've done OutlookExpress-to-Thunderbird
and
Incredimail-to-TBird-to-OutlookExpress...
but only on PC's.
Should be somewhat easy on a mac

http://www.hawkwings.net/2005/10/23/switching-from-thunderbird-to-apple-mail/
http://www.macosxhints.com/article.php?story=20061012031813924
http://www.google.com/search?hl=en&lr=&q=moveoutlook+express+mail+to+applemail&btnG=Search

That's interesting - I quite like your plan R C-R
- except that I'm 300 miles away from my Dad...

Well, you could have your dad email you his OE files as an attachment, do the export/import at your end, then email back the ~/Library/Mail folder (or just the various *.mbox folders from it) to him, which he could then import into his Mail app.

any other suggestions still really welcome

Don't overlook the comments in the macosxhints.com link Rick Van Vliet mentioned, which includes one that should avoid loosing any header info, assuming your dad has an IMAP account with some provider:

"Create a folder on your IMAP account.
In Outlook, move your email to this folder.
In Mail, access this folder. Leave the email on the server, or move it to a local folder if you wish."

Another alternative would be to extract the messages to (for instance) mbox format on your dad's OS 9 machine with a custom written OS 9 Applescript, transfer that to the new MacBook, & import it into Mail. There are probably OS 9 scripts for this around somewhere on the internet, but I haven't been able to find any links to them that are valid. Too old, I guess. 😟