don’t know, but I’d honestly wonder about the ability of UAs site to handle a CAD FFC on its own to purchase a ticket from the US website (ie priced in USD).

if it can, would probably be trivial to use it in combo with a USD one, as it would convert the CAD one the same way. However, I have my doubts.

Should be no harm to do a dummy booking and see how it works to apply them without actually hitting the final purchase button. But this might be a situation where dealing with an agent may be a better way to try this.