You can't 'combine' tickets without tossing them both and starting over, which would mean paying a change fee for each one.
You also can't fly a return of a ticket without using flying the previous segments... if you don't fly the outbound, the rest of the ticket will be canceled.

Its kind of confusing as to what you're trying to do, since you mention going from canada to NY but then thats not on what you want eventually. It would be easier if you listed what you currently have booked and what you want for cities/dates... but it sounds like the first situation I mentioned where you have two tickets and need to change both.