Originally Posted by
rankourabu
Answer is no.
I had the same situation, codeshare SFO-YYZ in P class. No way to add my NEXUS number during AC checkin, and wasting time going to AC checkin wouldve been longer than just sucking it up. Luckily the SFO intl gates security wasnt too bad.
You can just bring up your reservation on the regular AC site and add your TTN in the APIS/Passport information section. If the booking is made elsewhere, you cannot update your seat or FFN until check-in unless you use the alternate site upthread where you can buy/change seats before check-in.