You can achieve this using usergroups and rooms, something like this:
- UserGroup ABC can access, read and write in Room 123 so when you create the room you set the proper permissions.
- UserGroup XYZ can access and ONLY read Room 456 so you setup this room as Locked.
- Setup messages in Room 123 get reposted in Room 456, and obviously UG XYZ shouldn't see/access Room 123
So when a user gets a certain amount of posts you promote him/her to UG XYZ and when he/she gets a higher amount you promote to UG ABC. Or just promote to UG ABC if you let you Room 456 to be public to everyone or registered users.
Hope this helps.