I'm pretty sure you can do it. I actually used an eVoucher at a ticket window to get a new set of tickets for myself and my kid. The ticket agent asked if I wanted an eTicket emailed to my email address (it was on the eVoucher). It was properly sent. In addition to that, I was able to look up the ticket in my recent trips (it actually came up as the old reservation number) and I was able to add it via Passbook. Later on that option disappeared, but I was still able to look up my reservation on the app using the reservation number and my email address. With just that info I was able to pull up the reservation and then once it was up I was able to add it to Passbook again (I actually deleted the previous Passbook ticket and was able to add it again).
I'm pretty sure that when you make your reservation, your email address is attached to the reservation.
Of course I haven't tried this myself. You might find it doesn't work.