If I understand your question correctly, the way to do this would be via custom geo-coding the regions within the U.S.
From the KB below:
"If your geographic data does not fit into the built-in geographic roles, you can create new geographic roles and assign them to the geographic fields in your data. "
Here is the link providing the a step-by-step example and discussion of custom geo-coding: Custom Geocoding
Here is a Tableau user/blog who has created a handy tool to draw custom polygons on Google Maps to create custom polygon data files for the purposes of custom geo-coding (Please be aware this is an individuals contribution and NOT a product supported by Tableau). Creating Custom Polygons By Drawing Directly on a Google Map | Tableau and Behold!
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Tableau Online Deployment Advisor