I have read that it is a belief of Judaism that the souls of all Jews were present when Moses received the law at Mt.Sinai. This obviously cannot be the same case for converts. I have also read that a convert is special to God specifically because they did not see the vision of Sinai but still accept it. Is this idea true?
There are many ideas floating around and many of them have validity. I have heard it said that before G’d offered the Torah to the Jews, He offered it to the other nations. They all rejected it for one reason or another. But, although, the nations as a whole rejected the Torah, there were in each nation individuals who would have accepted the Torah but they were overruled. It is the souls of these individuals that eventually convert.