1). Enter the backoffice of your booking portal and create the extra rooms that will match the configurations in which you want to sell your private room. for example for Double Private room you already have a double room category in your backoffice, so you should create just a Single Private Room category.
2) Enter Manage--> Online portal and log into the xml settings for that booking portal. Enter Room map and click Clear and Refresh ( See it here) Your new rooms should appear in the white table "Online Rooms"
4) It is important to remember that there are 2 price options in this situation, because there usualy are two different prices in the HostelSystem settings: price for each of the two people and for the one person staying in the room.
As you can see price for one person is always in the line number one. Price for each of the two people is always in the line number two.(See it here).
While mapping your rooms you should check if there is a proper line indicated in the column Price in the table. For the Double Room the price should be taken from the second line.(See it here) For the Single Room the Price should be taken from the first line. (See it here)
Every time when one of those rooms will be booked the availability in the second one will close so there is no possibility for overbooking for such settings