Sub-Sub folders in WP 3.0.1 Multisite installation

I have WP 3.0.1 installed with multisite enabled and BP installed. When I add a new site I would like to use sub-sub folders, i.e,

I understand that having sub-sub folders was not its primary purpose but several users were able to do it with the Multi-Domains plugin.

Does anyone know how they may have done it?