You can already create a category only visible to a certain group by creating such a group and selecting the privileges in the ACP.
Administrators group doesn't show for category privileges
I have an admins-only section of my forum, and it seems that the administrators group is not an option when configuring privileges for a category. Is this a bug?
In the meantime, I created a "Site Administrators" group and can manually assign administrators to that group, which CAN be used when configuring privileges.
Not a bug, administrators can access any category. IF you want to have a admin only category just remove the privilege from registered-users.
Oh, that makes sense.