You may want to reconsider that. There is no indefinite article (a) in Greek. There is only the definite article (the). In the absence of the definite article (the) it is usually correct to assume the indefinite article (a).
However, in the case of John 1:1 another rule of grammar is in play. Even though our English translations all say "and the word was God" that is not how the Greek reads. The Greek reads "and God was the word."
The predicate of that clause (God) is first, or in the emphatic position, and is, therefore, necessarily without the article. (See
The Gospel According to St. John, Dr. Brooke Foss Westcott, John Murray, Publisher, London, 1908.) So, as the clause in question accepts the subject of the earlier clauses, it may well be translated "and the word was most emphatically God!"
The interesting part is that the JW Greek text is Westcott's text, and in other places they quote him as the leading authority on Greek.
So, the conclusion is that the JWs are right in that there is no article before "God" but they are wrong in assuming the indefinite article is to be supplied. They fail to consider the fact that "God" is in the emphatic position.