Originally Posted by
Majuki
I remember that this was only an option during an initial or renewal application. If you have an active membership, you'd have to get CBP to add your document for you.
This is my understanding as well at least as far as adding a second passport goes. You can update your passport online (as noted by mnbp) but to add a second passport, you either need to do it during the application process or go into the CBP GE office.
I'm thinking my situation is very much an edge case, though I have met the criteria to hold second passports on three different occasions in my life, so perhaps not.