"Every word of God proves true; he is a shield to those who take refuge in him. Do not add to his words, lest he rebuke you and you be found a liar" (Proverbs 30:5-6, ESV). In any case, you are not to add to God's Word since that would imply that we as a congressional party of humans can determine that more to the Bible needs to be considered canonical. Sure, accepting such a philosophy would inevitable have us assume that God actually isn't immutable, but as exhibited in scripture through countless exhibitions, He is God.