Ringtones, notification sounds and alarms are stored as sound files in separate subdirectories in /system/media/audio.
If you enable root access in your FP2, you can use a file manager that can use root privileges to navigate to those directories and copy/paste any sound files in them you like, and they’ll be there until you update to a new version of Android.
After doing the above, when you go to set your ringtone in the settings, your custom sounds will pop up in the selection menu - no special apps required.