euhm... it's possible, but not really easy since it has to be done over the internet :
create a vpn between the two servers, make sure the dns is working correctly (being able to ping the domains and the server names), upgrade the win2k server to latest servicepack + hotfixes, then run ' adprep' (which makes AD ready for use by windows2003 servers) -adprep /forestprep prepares the entire forest while /domainprep only updates the given domain. after that, run dcpromo on the win2k3 server to make it join the domain.
quite a lot of work though... have fun