Adding a password to a topic?

Can you add A password to a certain topic?

No it is not possible.

In order to accomplish what you are desiring you will need to create a hidden group, and give permissions for a particular forum, and the topic that would be private would have to go into that forum.

Then put all the desired people to whom you want to have access to that room, in that group.

It will then be accessed and seen to only them. Just make sure your permissions are set to be only viewed and read, and everything else to that hidden group.