Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
When you use a partner carrier (like CX) for the over-water segment of a one-way award with AAdvantage miles, the over-water carrier must publish a through-fare from origin to (final) destination, and that through-fare must permit the particular routing you desire. If CX's HKG-YVR fare does not permit a routing via JFK, then you cannot fly JFK-YVR on the same award.
To determine what you can "add on" after a HKG-JFK or PEK-HKG-JFK trip, you would have to find out what North American cities CX has a published through-fare for that permits routing via JFK.