Hi,
I have done exactly the same thing on weblogic 6.1.
Add the new servers from the console panel. you will notice <Server>...</Server> tags in the config.xml. you may choose different port numbers or different local ip for each of new servers. Also, in the ,<Server> tag, you can also mention the External DNS names for each of these server.
Other changes that you have to make includes setting up DefaultWebApp and log files for each of these servers.
Duplicate the script StartmanagedWeblogic.sh (and the stop script) by changing the
java command to suit each of these servers and start them one-by-one. hope this helps.
-Meera