If you use a sound where pitch is determined by a comb filter, like the piano presets, you can also change the divisions of the octave by lowering the Keytracking option on the second page of the filter. Essentially what @Jeanne suggested above.
If I were you I’d have a separate track just for the notes that need to be tuned manually for easier editing and workflow purposes. I tried to write some short melodies on just intonation and pythagorean scale on the A4 by manually tuning each note but that was no fun.
You can read about ratio offsets on the manual, a handy feature if you’re using fm.
Off-topic:
@littleprince96 if you have or record music with a persian santur by all means share a link or post it on the Current sounds thread™.