How do you assign a group of states to one value when assigning a geographic role?

I need to build a map that visualizes KPIs by region (a regional heatmap based on sales volume, for example) for a large data set where one field value represents multiple states. I tried assigning geographic roles to my 'Region' field, but I could only find a way to assign one state to the region rather than a cluster.

To illustrate the view that I need, I made an Excel table that assigns my Region field with each state and mapped it in Tableau, grouping the states to create map regions on the worksheet. I would union this table with my data set, but this would duplicate data from region level to each state--dramatically adding to the file size and causing aggregation problems.

Is there a simpler way to create custom regions on a Tableau map?