Hi Abid,
You can expose any topic by using Page Publishing. However this doesn’t allow replies, as the topic still lives in the restricted area.
Alternatively, you could also make your closed catergory into a sub-category. The parent Category could have access to everyone, and the sub-category could have access only to admins. Then you still have this closed category, but also a way of exposing them for all other users by moving them into the parent Category