Many financial apps support CSV export. I have been using, and contributing to, Money Manager Ex for AndroidMoney Manager Ex for Android. It is an open sourceOpen Source app and supports export to CSV and QIF exportformats.
I also wrote a Python librarylibrary for reading the database, so export to CSV can also happen outside the app itself, either on a PC or on a mobile device.
There are plenty of tools that will convert CSV, or other formats, to ledger syntax, which you can append to your book file.