You're pretty much on your own as no one on these forums has mentioned doing it.
That said, you may be able to do it with site aliasing, but as I wrote before you'd need everything in one account because the username must be the same for all domains.
If you need entire separate domains, it may also be doable (also all domains in one account) by having the admin make changes to the site-specific exim.conf file to point all domains to the same DocumentRoot.
Jeff