5.1. How do I import an archive into a new mailing list?
If your archive is in mbox format you can import it with
bin/arch <listname> path/to/your/file.mbox
If it's not in mbox format then you need to figure out how to convert it from whatever form it's in to mbox.
If you are importing an archive into a new list which currently has no archives, first copy your .mbox to
archives/private/<listname>.mbox/<listname>.mbox
which is where Mailman keeps the cumulative archive mbox for <listname>. Then you can run
bin/arch --wipe <listname>
to initialize the archive from the default mbox. If you are adding messages to an existing archive, you have two recommended choices[1]. You can run the first command above to add your messages and then append your file.mbox to Mailman's <listname>.mbox, or you can merge the files first and then run the second command with the --wipe option to rebuild the archive. In either case, you should stop Mailman before beginning and start Mailman again after you're finished to prevent Mailman's trying to add messages concurrently with you.
Caveat
Your mbox file must not contain any unescaped lines beginning exactly with 'From ' in message bodies as these will be interpreted as message separators. There is a bin/cleanarch script to help find and fix such lines if any. Run it with the --help option for more information.
[1] See this post for some more choices and detail.
Converted from the Mailman FAQ Wizard
This is one of many Frequently Asked Questions.