To add a 2003 server as a domain controller in a 2000 Forest you need to run ADPrep to prepare the domain for 2003. There is a good article here on the steps to do this.
Once you have done this you can run dcpromo on the 2003 server to promote it to a domain controller.
As of 2000 and newer there is not such thing as a backup domain controllers, they are all multi-master, however there are a number of FSMO roles that must reside on on server or another, and you may wish to move these to your 2003 server, or split them between the two. This article describes each role, and how to move them to another server.
Once that is done you will then have 2 Domain Controllers in your domain.