If you have access to Premium Support you would be better posting there to get a quick response from Nabble staff.
The only archiving facility Nabble offers is through:
Options > Download backup
However that is primarily designed for re-uploading to Nabble and considerable skills would be required to upload it to another service.
Your best bet is to use the kind of program I mentioned at:
http://support.nabble.com/ftp-access-td7592251.html
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.