QuickBooks Online supports multiple currencies while creating transactions. If you want to turn on Multi Currency Feature in QuickBooks Online, please refer the below article.


https://community.intuit.com/articles/1501407-set-up-and-use-multicurrency


Exchange Rate


Excel Transactions made it very simple to handle multiple currencies in your transactions. You just need to add one column named "Currency" in your data file.


"Excel Transactions" automatically fetches the Exchange Rate based on your Transaction Date ( Invoice Date, Payment Date, Bill Date & etc). 


If you want to override the exchange rate, just add one more column named "Exchange Rate" and provide expected value.