It is apparent that several of the pontiffs between the peace of Constantine 325 and Pope Gregory 600 made additions to the Canon. It is my understanding that the Canon was relatively fixed by the year 600. Christ obviously did not mention Cosmas and Damian, et alia, when the first Mass was offered. I am sure it was the custom of certain localities to add or omit certain prayers in the Canon.
Playing the devil's advocate, it took the Church 1800+ years to define the Immaculate Conception, and the act was done by papal fiat. Pius IX consulted the opinion of the bishops, "Is this part of the deposit of Faith?," but in the end he did it of his own free will, exercising personal, extraordinary infallibility. One could argue that Pope John did a similar thing with the Canon. It seems to me the argument of St. Joseph being added to the Canon seems to revolve around the question, was John a pope or not, rather than the question, does the pope have authority to touch the Canon.