You can import ringtone files to the app using the same method as importing songs. Email them to yourself, or place on cloud storage, etc. Those files are in m4r or m4a format and that is a recognized format for the app.
But, I can find no current way to get access to the files for the ringtones you purchase from iTunes.
Apple provides no method for apps to access those ringtone files on the device.
There used to be ways using iTunes on a MAC or PC, but Apple removed all options for that with iTunes 12.7.
I do see PC/Mac app utilities that claim to be able to get ringtones from your device, but they are not free.
Perhaps there are some expert users of the forum that could point out a way to export the ringtone files.