Hi Crystal,
While ListMail does not currently have a refer-a-friend feature, you can add any HTML code you want to the Custom HTML "Signup Confirm" page. This could definitely include an HTML form posting to your refer-a-friend script. Usually your script will mention the code you should enter.
To set up a custom set of 6 Custom HTML pages click "Custom HTML" then "New". Set up your pages, name the set and save it, then go back to "List Settings" and choose the new set in the "Custom HTML" select box.
You should be able to do any and all of this on either your ListMail domain or another domain, or a combination of both.
Please let me know if I can be of further assistance!
Regards