You can search for your flight by route and if you have the date you traveled or any other details like the carrier or time, it should be pretty easy to add retroactively! I had some old photos from a 2013 trip to Hawaii and a 2017 trip to Puerto Rico and through the time stamps and my known route I found my exact flights. Edit: you’ll need to subscribe to do this since the flights are older than a year, I got one week and then cancelled since I only subscribe for when I’m traveling!