Michael Jackson had a good vocal range and that is because of his long time spent singing. He had been singing since he was five years old. Having that much experience allows someone to have a good vocal range. If someone wanted to develop the same vocal range as Michael Jackson, they would need to plan on practicing for many years.
Also, the person would need to plan on hiring some voice coaches in order to help train their voice and vocal cords in order to be able to get the range that Michael Jackson could. If someone could do that, then they could plan on making it big in the music industry. Michael Jackson would probably be very proud of someone working that hard on their voice.