On converting old archives:

The easiest way to do conversions is if you have an old mbox file, so what I'd suggest is that if you don't have one, you make one using what you've got.

I've never used to Dreamhost, but I'm assuming what your client has is a bunch of HTML archives. There are plenty of ways you can download HTML archives and try to convert them back to mbox files. I've recently taken to using Perl's WWW::Mechanize for slurping up things I want to scrape off web pages, but really anything will do and I'm sure there are plenty of other packages available for web scraping. Once you've got the information from each message, you just need to reformat it into a text file. A google search should tell you what an mbox file looks like, or you can send a couple of test messages through your list to see what mailman makes.

Once you've built up the mbox file, you need to let mailman parse it to generate the HTML archives. Since you've got a fresh install, you won't have any existing mailman archives (well, barring a few test messages that you probably don't want) so you can just copy this file into $MAILMAN/archives/private/$LISTNAME.mbox/$LISTNAME.mbox where $LISTNAME is of course the name of the list. (If you do have messages there, you'll need to concatenate the files together so that the posts are in chronological order.)

When you've got the archive in the right place, you'll probably want to run $mailman/bin/cleanarch to make sure it's ok, fix any leftover problems, then $MAILMAN/bin/arch to generate the archives.

Make sure that that mbox file is writeable, and when you start up the list, Mailman will add new messages to the end of that file and generate further archives on its own.