I probably explained things badly. The answer is hidden in the second paragraph of my post:
In essence what you are doing in setting up a Nabble archive is to subscribe your new Nabble application to your existing mail list. Nabble then posts any messages it receives as a new subscriber to a dedicated Nabble forum.
so Nabble will only get mail from the time you subscribe it to your mail list. It does not attempt to retrieve any mail backlog.
There are many forms and format of archive on other mail list servers and the requests for backlogs to be installed on Nabble are sufficiently rare for it not to have proved worthwhile working on automating the process. Perhaps more importantly, sucking off an entire backlog put an unacceptable load on some servers and not be at all welcome by their operators. It's not inconceivable that it could be sucked into a DDOS attack if the process was available for any Nabble user to initiate automatically.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.