Would it be legal to publish a copy of the Constitution with an extra article that is not part of the real constitution, with no indication that it was added, i.e. intending to mislead readers?
The US constitution is in the public domain. Anyone may publish a version of it, including an altered version. No US law forbidding publication of an altered version would itself be constitutional -- the First Amendment would prevent such a law.
However, if an altered version were sold under such conditions that a customer might reasonably believe it to be an unaltered version, that might be false advertising, or perhaps fraud, because the seller would be deceiving the customer as to what the product is.
Merely publishing it as provided wouldn't be actionable or a crime, but if it caused economic or legal harm to someone, or was used by someone knowing of its alteration in a legal matter, this would probably be actionable by someone in a manner that would depend upon the way that the issue presented itself.
This Has Happened Before—The Government has Even Done it
According to a law review article by Jol A. Silversmith, “Well into the second half of the nineteenth century, some textbooks, state compilations of law, and even on one occasion a compilation of law published under the auspices of Congress erroneously included” a proposed-but-never-ratified “titles of nobility amendment.”
It is not Illegal to Misquote the Constitution, in and of Itself
To this day, there are some people who mistakenly believe that the TONA was ratified, or that various other amendments never properly were. There’s no law against being wrong.
It’s also not against any law, for example, to give a bogus quote of the Constitution as a joke. (It’s right there in the First Amendment: “Congress shall make no law regarding an establishment of religion, or prohibiting the free exercise thereof, or abridging the freedom of speech, or the press, or making me work on my birthday.”)
What You Do with Your Fake Constitution Might be a Crime
For example, if you claimed to be selling authentic copies of the Constitution, and you knowingly sold your customers something else, that could be fraud. Or, if you printed up a fake copy of the Constitution to show clients and deceive them into paying you to file a frivolous lawsuit, you could get in serious trouble for that too.