You can do it from your current private list archive.
Note that Nabble itself is a mailing list. So, you can simply add your members to the forum as subscribers (they need to confirm it), then they can either access your forum as a regular forum, or they can access it as a mailing list. This saves the trouble of having to have a mailing list and a third-party mailing list archive. For more info, refer to:
http://n2.nabble.com/help/Answer.jtp?id=46