Hi Anand -
I don't want to be the bearer of bad news, but Tableau isn't really built to be an ETL tool - which is how it seems you're trying to use it in this scenario.
You can export a crosstab to Access via Worksheet | Export | Data and then connect back to the MDB as a data source. However, your real challenge will be trying to display so many "marks" (rows) in the Tableau crosstab before you export it.
Tableau is designed for data visualization / analytics, not for displaying millions of rows - realistically anything over a couple hundred rows of text is of questionable value in terms of spotting patterns, trends, etc.
As you display hundreds of thousands of rows in a crosstab, you'll see Tableau consume more and more RAM. You'll eventually run out of RAM in the Tableau process -- probably before you get to a million rows.
You might be able to get there by exporting smaller batches of rows from Tableau to Access and then "knitting" them back together in Access, but I think it would be more trouble than it's worth. In your shoes I'd probably look to a true ETL tool.
Probably not what you wanted to hear, but hopefully I can save you some grief.