I don't know about adding verses, but there is absolutely no doubt that KJV translators changed words, added words, and translated the same words differently in various places. And the KJV is in no way a direct translation from the original manuscripts.
The question is why. Were they incompetent, were they trying to mislead or did God direct them in their actions.
That is a fair assessment of the KJV, I think.
But... your question could be asked of any of the seriously undertaken, more modern translations today. I firmly believe that God (when asked) provides guidance in the translating of scripture... and THAT is where the "inerrancy" of scripture comes from... NOT from believing that every word, punctuation mark, jot and tittle is EXACTLY the way the original writers put it on paper.
I don't believe that God worries about whether it was 70 or 72 folks crossing a desert, or any of the other myriad "discrepancies" that both pro and anti KJV-only folks have dug up.
Does anyone really believe that God wants us to be spending our time here on earth trying to prove who has the "rightest" version of (cue the deep bass voice) "THE WORD OF GOD"? (and I know "rightest" isn't a word...but it fits this discussion)
I believe God has preserved His word in many different translations, because the message He intended us to have is there... in ALL of them.
If there are any questions, or confusions, people do exactly what the KJV-only folks insist on.... they STUDY... multiple translations, commentaries, Greek, Hebrew.... and God/Spirit provides the understanding.
That's in no way different than having to study other texts to understand "olde English"...
I think this silliness has gone on long enough... but that's just me, I suppose.