Is there a way to group foreign customers (ie. non-US companies) into 1 category in an AR Aging Report?

What will be the basis of being a foreign customer? Country in address? Category? a custom field? Currency? Will the open balances still be shown using the base currency (I presume)?

country in billing address.

If it were for me, I'd add Country Billing Address as a Filter so I can filter USA alone or foreign countries as needed. Or I would create a custom field that indicates whether a customer is foreign or not (say, values are US and Foreign) so I can add this field on the Sorting tab of the report. This way the customers are 'grouped' into two (Foreign first then US unless descending is checked).