The KJV translators did not intend to add to the Word of God. They did their best, but all they had to work with was a handful of copies of the Greek manuscripts of the New Testament books. These were very late copies dating from a thousand (!) years after the New Testament was written. In a few sections they had no Greek manuscript at all! Instead, they had to rely on the Latin Vulgate's rendering of what they thought must have originally been in the Greek!
Through the providence of God, many more Greek manuscripts had been preserved and were subsequently discovered—in fact, more than five thousand of them. Some were very old indeed, dating back much farther than the relatively few the KJV translators used. Some of the Greek manuscripts date back to the four hundreds and three hundreds—even to about a.d. 200. These ancient manuscripts were more reliable and accurate, not being corrupted by errors made during countless times of copying, such as occurred with the late manuscripts used by the KJV.
As a result we know today, with a high degree of accuracy, what was in the original writings.1 Uncertainty now exists in only an infinitesimally small part of the New Testament (the difference would be comparable to that between "don't" and "do not" or "street" and "way").
Some examples of verses that the KJV added to the Word of God, even though it did so unwittingly and in all innocence, are