To set up a sublist to an ezmlm list, just use the ezmlm-make -0 mainlist@mainhost switch. This will configure your list as a sublist to the ‘mainlist@mainhost’ mailing list. Since the main list may not use the ‘Mailing-List’ header, you must identify another header that the main list adds to all messages. See the ezmlm-reject(1) man page for examples. Next, edit DIR/editor of your sublist and add a -h Listprocessor-Version: option to the ezmlm-send(1) line, but replacing ‘Listprocessor-Version:’ with your mainlist header.
Now your list will accept only messages from ‘mainlist@mainhost’ and with the header specified.