M
You have to keep it in Revelation .To understand Revelation .Rev.22/18-19
19And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
I think you are confusing adding with verses in Revelation referencing to other verses of the Bible.
Example:
Rev 16
15Behold, I come as a thief. Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame.
You can see here where Revelations quotes a Christ in another book so we can confirm the word. This is very different from adding to the book of Revelations. By this we know to go back and see what Christ himself taught. This is not adding to Revelations, rather getting confirmation in the word. Big, big difference. The book of Revelations doesn't contradict the rest of the Bible, it confirms it.