Regarding own repository of directadmin files, you should set up a script which would mirror files1.directadmin.com
and then you should change IP for files1.directadmin.com on every server, on which you'd like to connect to your private repository, like it's said here:
or setup a recursive dns and add files1.directadmin.com there pointing to your own IP. Consider to add the IP of your recursive dns into /etc/resolv.conf
Regarding own repository for CentOS, if you want to host rpms for CentOS this is not the right place for asking how to do that. You should refer to official documentations from the developers, or try your luck in CentOS mailing lists.