I believe you can do it manually. You'd need to copy the keychains, the folders in ~/Library/Mail and ~/Library/Mail Downloads and Mail preferences from your ~/Library/Preferences.
This can be faster depending on how much mail you've got.
And make sure you've got a good backup AND be aware that messing with mail is a bit different from other stuff, in that changes you make to an IMAP account will propagate into the cloud where the mail is stored on the server. So if you screw up you can appear to Gmail or whatever to be making changes that it should pay attention to...like deleting stuff. This can create a bit of havoc, so be careful if you decide to go this route.
Migration is a safer route, as is recreating boxes from you IMAP accounts (although that doesn't help with locally stored stuff).
Rob
