I have to totally disagree. Strom is right. If you own some legal rights that were passed along the capitalist byway, then you can decide what is canon "officially."Just Old Bear wrote: When you own the rights you can make anything canon that you want to. That's a reality I had to come to terms with 4 years ago.
Canon as far as fans of the spirit are concerned, however, would be a totally different matter, and the original author is the only true source of this canon. The writings of the guy who gave birth to the Hyborian vistas are the sole authority. The world is nothing more than those stories. When Howard took his life, the possibility of new canon ended. People who own Van Gogh paintings can't start making pastiche paintings and then call them "canon" or "original" works of Van Gogh art. Art, in spite of capitalism, does not work that way.
All those other Conan stories, games, and visuals can be fine and dandy, but they are still glorified fan fiction.
And, if we are using the religious analogy: The Islamic scriptures feature Jesus, but I doubt Christians consider that "canon" simply because the same character is prominent. Also, the Hebrew Talmud and Midrash are weighed differently (although the two aren't always considered hierarchical in many traditions nowadays). Also, Hindu scripture fits into various levels of "heard scripture," "traditional scripture," and "hearsay scripture," very few pieces falling into the paramount canon. Just smushing it all together as "canon" is very irresponsible.
Look at the Highlander movies, for crying out loud. Look at Sherlock Holmes.
Let us take the dirty Marvel universe as an example, where every year editors have to clear house and start deciding which stories should be canon and which forgotten. In that case Marvel owns the characters. Many writers and artists take their perspectives on Spiderman, creating who he is. Stan Lee and Ditko may have created the guy, but he was always a group-owned and group-written character. A company with legal rights goes through and decides which writers count, and that is alright. If we were talking about Spiderman, sure, legal rights are the same as artistic rights.
But, Conan was not developed that way. The Mongoose Conan RPG is Fan Fiction. Legal fan fiction. But fan fiction. Good fan fiction. But fan fiction. They just have the government's patent office on their side... But (did I say this yet?) fan fiction.