As mentioned, it has to do with capacity requirements for safety. A room that has a capacity for a number (two on your case), cannot be exceeded as part of booking. There are ways to manage around this, when booking (in the selection of rooms). That said, technically most couches can be converted to a third (in a balcony room). There is also a big difference in billing (as you may know), a third person in a room almost always pays a significantly lower price.
Not sure how you booked, if you used a TA or PVP and if you informed them of your potential plans, they should have been able to assist for the best possible solution. If you booked yourself online, this is a great reason to use a TA or PVP. Their knowledge is free, and in this case would have saved a ton of money and aggregation. Have a great cruise.