When you create a Nabble archive of a mail list, what you are doing is subscribing your new forum to the mail list. It will then post any message it receives to your archive. Nabble has no automated mechanism for grabbing previously posted messages.
If you want to add previously posted messages to your archive you will need to post an MBOX archive of past messages at:
http://support.nabble.com/Mbox-import-f7590796.htmlfollowing the instructions at the top of the page and then Nabble will import it for you.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.