What if I was willing to pay the full difference? If the 5-day Canadian discount pass has a $300 value (for arguments sake) and the 4 single day tickets are $450, could I pay the $150?
I don’t think it is possible or worthwhile. You should buy new tickets for the current trip and save those multi day tickets for the future. Disney has changed or will change rules such as expiration and seasonal blockout. So those old tickets will be worth more to attend high season in the future.
The general rule of thumb is that Disney will never refuse to take more of your money. Technically speaking they won't break up a ticket like you are suggesting but it never hurts to ask and you might get lucky and catch a CM who is willing to do it. You will lose the CAD discount and then on top of that you will have to pay gate price of those single tickets.