You can only do this by calling in; it cannot be done online. It would probably be easier and less hassle to just call to do it all instead of doing room first online. It's called a ticketless package, and all package rules apply. Once you have your package reserved, you can add your tickets.
One note: you may want to check on the expiration of those Canadian tickets. I know all U.S. based tickets expire either at the end of the year purchased or the following year even if they are not used. You can still use the purchase price paid to buy new tickets, but they will only give you what you paid for them (I think) toward new tickets.