you could however you will still have to pay the higher of the US and Canada fares. which in this case would be the US fare and thus not worth it. if the cities in question are YVR and SEA it would be feasible to drive/fly up to YVR and have the tix issued. this is the case unless OW specifically allow ex. Canada fares to be sold in the US at C$ rates??

I believe (as is the case with * Alliance) that in Canada you can have a ticket issued at the fare of the origination point. With *A I have done this and had an RTW issued ex. India with the INR fare.