Suz:

It's never a good idea to do sales analysis by breaking out sales in the Ledger entries.

You have the correct solution, use the Sales Analysis module to break out sales by CAD vs USD customers, either with the ^ in the Customer Code or by using the customers Control Account Code.

You should also have the Multi-currency module to manage the USD conversion for you.

Otherwise, for every USD sale you would have to change the Category Code on every item so that the USD sales go to a different GL Account.

This is the way all accounting software, that I've seen, works. Your Adagio Consultant can help you work through this problem.