Originally Posted by
Porker
This method still seems to hold true except I still cannot seem to find a place to add a passport for my companion. Now said companion is a minor, and thus had to be added over the phone, so this may have something to do with it.
The only way I know of is to go to Flight > Manage Reservations, then enter name and PNR. There should be a link for Add/Edit Passport information right under your name unless that's what you're already trying to do.