Even though Google Photos storage won't be free anymore going forward, the service is still among the best options when you want an almost automatically organized personal media library. But Photos falls short when you do want to do some organization on the go, before uploading your files — you used to only be able to add photos and videos to albums when they were uploaded. Thankfully, that has changed. You can now add content to albums even when you're offline.
Adding photos and videos without backing up files works both for existing and new albums. The changes you've made will be synced to Google servers as soon as you're back online or back on Wi-Fi, depending on your data saving preferences. The process itself is smooth and just like you'd expect it to be when you're online. You simply select content, add it to an album, and you're good to go.
Note that I'm offline and there are pending uploads.
It looks like the change is already live in most recent versions of Photos. If you're still unable to add photos and videos to albums while you're offline, you might have to wait for a server-side switch to flip. You can also try downloading the latest version of Photos from the Play Store or APK Mirror, but your mileage may vary.