I have now completed a procedure for the Mail transfer - there are some minor (in my opinion) issues - see footnote, but here it is:-
Procedure that I used to move MAIL data from Entourage 10.1.6 on old G4 Power Mac (old computer) to Outlook (Office 2011) on new Mac Pro
(apologies if this appears very low level but I found it easier to include every step in detail - and apologies for any typos).
Warning no guarantee I can only confirm that it worked on my system for most message types (see footnote).
I recommend taking a backup of your Home Folder in Users on both computers before proceeding in case anything goes wrong.
On old computer open Mail application (included with default Applications)
Skip the import mail stages, until you end up with an almost empty window. (You may see empty Inbox and Sent mailboxes at LHS).
Select File Import Mailboxes and from Import data from select Microsoft Entourage and click continue.
Select the Items to import (I selected everything) and click continue.
Note the location of imported mailboxes and the name of the folder (I am assuming it will be called Import ) and click Done.
Select the Mailbox Import and leave it highlighted.
***From the top of screen menu select Mailbox - Archive Mailbox.
Select Desktop as the destination and click Choose.
A folder will appear on the Desktop called Import. You can now quit Mail.
To avoid confusion later on I suggest you rename this folder MailTransfer.
Check that this folder contains your expected subfolders and that they are not just default 4Kb items.
If they are not of sensibly large size something has gone wrong with the archive.
If this problem occurs delete the folder from the desktop, re-open Mail, and repeat from *** at the start of this paragraph.
Transfer this folder to your new Mac Pro desktop (I used a USB memory stick).
On the new Mac Pro open the Mail application (which may or may not be empty)
Skip the import mail stages (which will appear if it is empty).
Select File Import Mailboxes
From Import data from select Apple Mail then click Continue.
Select the Desktop folder MailTransfer and click Choose.
In the Items to import list make sure all items are ticked and click Continue.
The folder will be imported into Mail. An additional mailbox will appear named Import in the list on the LHS. Click Done.
(Note that the name Import is the default name of the first imported mailbox.
This is why I changed the transfer folders name to MailTransfer earlier.)
Mail now contains the imported messages from MailTransfer in the mailbox Import and ANY PRE-EXISTING content you may have had in Mail.
(Note - everything in Mail will be included in the next stage.)
Now close Mail.
Open Outlook and select Mail from bottom LH corner pane. - A blank Mail screen will appear.
From top of screen Outlook menu select File Import
From Begin Import What do you want to Import select Information from another application. Then click right arrow.
From Choose an Application -What would you like to import select Apple Mail. Then click right arrow.
On Import Items ensure all items are ticked and click right arrow.
Your mail will be imported and appear as Mail Import in LHS pane .
Click Finish - The procedure is complete.
The mail is now found listed on the LHS under Mail Import Import
You can then sort all your mailboxes as/if required !
Footnote: I found a few formatting corruptions but most messages seem OK.
However I have also noted that some attachments transfer all the way
to Apple Mail on the new Mac Pro, perfectly correctly, but do not fully display in Outlook Mail.
I think it must be an attachment file type problem but I havent solved it yet.
This was why I delayed this post.
However if there is something vital you need to refer to, that doesnt open fully,
then there is always the option to open Apple Mail and view it there.
In my case it is a very small number of items affected,
so I hope it does not cause you a significant problem.