[CentOS] Adding Update repository at install time

This is easy with Fedora, as the various 'common' repos are listed and 
all I have to do is change the URL....

Anyway, I am doing a netinstall of Centos 6.3 i386 from my local repo.  
I also have the updates repo.  How can I add the updates repo so that I 
get it up to snuff right away (like I have been doing with Fedora for 
quite some time)?

thanks.