You'll find the ExMerge utility on your Exchange 2000 CD, it comes with a very good readme file that's quite detailed. You can also check out this KB article on Microsoft entitled "Migrating to Exchange 2000 using ExMerge":
http://support.microsoft.com/default.aspx?scid=kb;%5bLN%5d;Q327304
or this one called "Some questions and answers about ExMerge":
http://support.microsoft.com/default.aspx?scid=kb;EN-US;265441
that should get you from A to B. The biggest issue with ExMerge is making sure the account you're running it under has full Send As/Recieve as rights on all the mailstores - I can't say how many folks have emailed me saying DiRT was broken because they couldn't backup or restore messages with ExMerge - about %99.9999 of the time they haven't granted the permissions properly. The help for ExMerge links to a KB article explaining why this is necessary (short story Domain admins are explicitly denied the right in Ex2K).