This would be very hard to achieve, and in my opinion, it would actually have a negative effect on the customer experience.
As you may have noticed, our checkout system will only allow you to charge in 1 currency type. If you are charging in CAD, the customer will see their total in CAD always, even if they are paying in USD. The USD is converted when they use their card, and the conversion would be at the same rate as a Currency Converter Apps would use.
This keeps your store's charges as transparent as possible to the customer. If we were to change the conversion rate on the app, the customer would see a different price charged than they see while shopping. This would cause confusion.
To answer your question, it could be possible to set a flat conversion on currencies instead of dynamic updated rates. You should get the advice from your app developer if you wanted to do this.
Otherwise, the only other way I can think of doing this is to have custom code made to have prices mulitplied by a specific amount when a specific currency is chosen. This would require a large amount of coding, and I would suggest an Expert if you are not familiar with coding.
I hope that information is helpful.
A startup Haloy https://www.haloy.co/ is trying to solve this problem! Haloy gives Shopify store owners locked exchange rates that are valid for any amount of time and displays FIXED prices in your customers' home currencies. With the locked exchange rate, both your customer's purchasing price (in their local currencies) and your revenue (in Canadian dollar) will not be affected by the ever-changing real-time exchange rates. Haloy will automatically process each foreign sale in the locked exchange rate, so you don't need to worry about it at all after integrating Haloy's app into your store.
Haloy is currently developing its Shopify app and collecting an early access list. You can go to their website (https://www.haloy.co/) to fill in your email! Let me know if the answer helps :)