If it builds, then two more :)
Because a system with three identical planets in orbit is more stable than two (see "Lagrange points").
But frankly, I don't see the point. Better to expand to other stars (and technically, it's probably no more difficult than bringing an entire planet from somewhere) than putting all your eggs into one ... well, all planets into one star system.
I will not waste time on clarifying technical problems, such as where to get 6 * 10 ^ 24 kg of matter and at the expense of what energy to accelerate it to 30 km / sec, the short answer is no. The appearance of such a massive object inside the solar system will lead to a violation of the orbits of all other planets with unpredictable consequences. By the way, there is a program that allows you to simulate such events or, for example, place a black hole near the Earth. There are many examples on YouTube. http://universesandbox.com/