Would this be possible to do in a mod? Or has it already been done?
Is sub forums like this?
It depends on what you really want to do.
If with "sub forums" you mean feline's mod, that one will take a single database of users and posts and split it in multiple forums (AFAIU).
Instead if you want to "connect" different databases of users and messages, than it depends on many other factors (though I think there is something around...no idea where...).