I can't find nor have I ever seen a utility inside of XG Works that specifically allows you to pre-define custom scales. You can in the XG edit mode edit any existing track down to 1 /24th semitones. You can also insert a new semitone parameter at the beginning of any track and play from there with the exact pitches you like, but you would have to insert any semitone changes that you want at the specific interval that you want it to occur, much like doing a CC change or any other edit in a standard midi file. Also, you will need an XG synth or module because none of the XG functions work with non XG devices.
I don't use XG works very often any more as I now have Sonar and I don't have any XG boards at this time ( although I may at some point get the XG plug in for my Motif ). Still, the one reason I'll always keep XG works is that I can step record and insert chords with it using any of the Yamaha arranger patterns or others converted to Yamaha .sty files. The software recognizes all of the variations in a style and many chord types, including chords with bass inversions.
AJ
_________________________
AJ