Happy to help. The OpenSong XML format is described on the 'OpenSong' site
OpenSong | File Formats , this is a separate product that shares the OpenSong XML file format with OSA.
However, my experience with OSA Options > Songs > Import a song is that it supports
Text formats - Text (.txt), ChordPro (works for .pro, .cho, .chopro, .chordrpo)
Image formats - Image (works for .gif, .png, .jpg) and PDF(.pdf).
OpenSong XML format - OpenSongApp song (.ost), Desktop OpenSong file (no extension)
If you have text based files in songBookPro then export to ChordPro is perhaps an option. ChordPro has less information than OpenSong XML but it has the song lyrics and chords and importing those is lots of the work!
You could import them one at a time but I found a work around for importing lots of chordpro files. Put the files directly in the opensong/songs folder using a file manager, restart OSA, find a chordpro song in the song list and open it - this does a convert on the fly. Please note that this confuses the song list into showing the converted song and the original, do a restart of OSA to sort that.
Of course, import can go wrong!
Sarah