Is that possible at all? If not are you planning such functionality?
No that's not possible. And I don't have plans on adding that, but I will put it on the wish list.
You can have sub-forums of the main group forum but not a sub-forum of a sub-forum.
The general suggested layout is this...
Main Groups Category (category node to hold all other categories)
Group Category 1 (sub-category node which is selected when creating a group)
Group 1 Main Forum (forum node created when creating a group)
Group 1 Sub Forum 1 (sub-forum node created when creating a group sub-forum)
Group 1 Sub Forum 2 (sub-forum node created when creating a group sub-forum)
....
Group 2 Main Forum (forum node created when creating a group)
Group 2 Sub Forum 1 (sub-forum node created when creating a group sub-forum)
Group 2 Sub Forum 2 (sub-forum node created when creating a group sub-forum)
....
Group Category 2 (sub-category node which is selected when creating a group)
Group 3 Main Forum (forum node created when creating a group)
Group 3 Sub Forum 1 (sub-forum node created when creating a group sub-forum)
Group 3 Sub Forum 2 (sub-forum node created when creating a group sub-forum)
....
Group 4 Main Forum (forum node created when creating a group)
Group 4 Sub Forum 1 (sub-forum node created when creating a group sub-forum)
Group 4 Sub Forum 2 (sub-forum node created when creating a group sub-forum)
....