Kerryn,
Unfortunately, due to internal security concerns, this is not yet a feature. There is the potential for abuse and unexpected issues (server crashes, etc) when ListMail owners start giving accounts away for free or cheap. Many restrictions and access controls will need to be implemented. I hope to be able to provide this one day. For now, if you trust the user, you might set up an additional installation/database of ListMail for them to use on your site.
Regards