Hi Sally,
The indefinite article (a or an) is required only before singular countable
nouns that are not preceded by the definite article, a pronoun (for example,
her), and numerous other words. This is fairly complex rule. If you want to
create a macro that will correct "dog" to "a dog," you're going to need some
very complex code that can access a database of all the singular countable
nouns in the English language. Even if this is not beyond the scope of Word
macros, you should know that you will be facing a formidible task.
As for Graham's definite opinion against adding a comma before the serial
and, I would like to add that the practice in the UK is not to add this
comma, but in the US, some very influential authorities, such as the Chicago
Manual of Style and the MLA Style Sheet, say that the serial comma should be
added. Thus, the prevalent practice in US English is to add the serial and.