a chorded synth where each oscillator/voice plays a note of the chord..so when you hit one note the two/three osc play for example c,e and g.. if you use this then to remain in a key you will need to set up a chorded synth for major,minor,diminished,augmented...it is actually quite limiting..what about sevenths,color tones etc??
you need to set up one for each type of chord..
for example any major scale would need a major as the 1 chord
2 minor ,3 chord minor,4 major, 5 major, 6 min, 7 dim..
so u could use the major synth for 1,4 and 5 and minor for 2,3,6 and diminished synth for 7..
anyway ..
to make that 5th note 'd' a seventh..you have to decide first of all what key u r in then which chord you want to play when the melody plays a 'd'..
from your melody notes i am assuming that you are in 'C'..
and assuming that you are in 4/4 time..and assuming that you only want a chord at the first note of the bar assuming that they are 1/4 notes..
then on the second bar D F F C..you want a chord to go with 'd'..
you could use a G..the dominant 5 chord..G,B,D..and if you want a seventh then F# or F..
you could choose to use Dmin7(D,F,A,C) or maybe Emin7(E,G,B,D) or Cmaj9(C,E,G,B,D) or B dim7(B,D,F,G#)
try various chords and voicings till you find what u like..
good luck..