By SQL Database, are you referring to SQL Server? If yes, then on SQL Server you can set up a linked server to the Access DB. You can then create a view to union the data, and use the view as a data source on Tableau.
Yes, SQL Server. But the problem is that I don't have Admin rights to the server. I can't set up a linked server; and getting this done would be kind of an impossible task. Any other way of doing this in Tableau?