You need to consider UNION option. I am not sure if your primary data source is. As long as both are access I think you can UNION the data to get desired results.
The primary source is a big database coming from a data mart, and the second one is an access/excel table that I would create to supplement the first source with an additional category. The article says that both tables must come from the same connection and have the same structure in order to make a union. In this case it would be impossible to have both tables in the same connection.
Can you think of any other way to get desired results?
Did you have time to check the issue above? I'd like to add a new category to my bar chart, and I can't make it work.