It is forbidden to add anything to either the Written or Oral Law Deuteronomy 13:1 "You shall not add to it"
Would you think of adding anything to the "Declaration of Independence?"
Of course not! This important document, written many years ago, served as the basis for the Constitution of the United States.
Even today, we use its messages and ideals to formulate laws.
No one would try to change its wording or content.
How much so, would no one dare to add to the Torah, which was not written by man, but rather contains the words of HaShem!
We are forbidden to add anything to the Torah.
For example, we may not wear five Tzitzit instead of four, nor may we make up new Mitzvot on our own.
I know this is old, but I searched and found it. I figured I would make a comment. I read there is a disagreement among the Rishonim whether one who adds additional strings to the Tzitzit is transgressing the prohibition of Bal Tosif, or that it is permitted to add as many strings as he wants, unless he adds strings of a different type. I read that the Shulchan Aruch ruled that if one added strings even from the same type it is invalid. The Mishnah Berurah quotes the GRA who agrees with the Itur that it is permitted to add as many strings as he wants as long as they are from the same type. The Mishnah Berurah adds that the Artzot Hachaim quotes another few great Rishonim who agree to this. I point to pictures of Rabbi Chaim Pinchas Scheinberg ZT”L and Rabbi Dov Kook who are both pictured on the internet wearing multiple Tallitot, and we were taught never to do that.