xapk is just a zipped apk and a data folder. rename it to something.zip and extract it with Material Files if the stock file manager cant do it, but it should be able to.
Then install the APK and move the data folder to /storage/emulated/0/Android/obb/
So if the folder in the xapk is com.example.example then the path should be
/storage/emulated/0/Android/obb/com.example.example
after the move.