Within each order record are three values you need to look at. There is the currency the order was placed in, the value of the order, and the currency exchange at time of pourchase. I believe, although I may be wrong, that the order_value is actually the value in your default currency. You would therefore need to just use this, along with the $currencies->format($order_value) function to show the order value in your default currency.

Where you place this code depends on what you want to do with it. As the customer has placed the order in CAD, I would probably add it to the bottom of the order totals, so that you have both totals there in front of you, just in case the customer ever queries it, and you need to see how much they actually paid in their currency.

Absolute