I booked as one-way flights as well. I believe that as long as there is 1 seat available on the plane, you can add your companion. Obviously, if there are no seats (totally sold out flight), then you cannot add them.
It was just a little frustrating to have to make a phone call for something I should be able to accomplish online. But it all worked out. I don't think that I'll see that leg go lower in price. The dollar value was $102. However, the flight to MCO could also go down and then I would have to call again.