There is no need to ticket this in Canada. Any One World RTW fare priced and bought in the USA will cost the same amount as if purchased in the country of origin for the first segment.
In my case I used a US travel agent who sold me a RTW ticket based on prices from the Netherlands. Obviously the proce was converted from Dutch Guilders to USD.
My suggestion is to call Americans Round the World desk and ticket through them. If you still have problems I can e-mail you my travel agent name and number who would be happy to help.
rich