Many RTW fares only require you to finish the routing in the country of origin, but not the city.
However, Canada and the USA are not the same country for RTW purposes. The RTW fares ex-Canada are cheaper, and airlines are on the lookout for people trying to pull a fast one.
What is perfectly permissible is to buy a one way USA-Cda to start your RTW here. Then within the next 12 months, you can use the last sector (conveniently USA-Cda) and start another RTW.