Well, Nabble can't duplicate that format exactly, but you can get pretty close by use of a combination of Nabble applications. Experiment with combination of these application types for your forum and sub-forums.
Mixed
Page with topics grouped by sub-applications. If no sub-apps are available, a normal list of topics is displayed.
Category
Page with a simple list of sub-applications and discussions.
Board
Page that groups sub-applications and discussions.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.