As with major scale tone chords, it is possible to add 9ths, 11ths and 13ths to chords built on the notes of minor scales. Shown below are scale tone 9th chords built on an A harmonic minor scale. Notice that some of the chords are no longer straight minor, dominant or major 9ths, but have more complex names because of the alterations caused by the raised 7th degree of the harmonic minor scale (in this case, a G# note). These chords may seem confusing at first and may take a while to learn, but in the long run it is well worth it, as they will help you improvise better and more easily in minor keys.
When harmonizing melodies in minor keys, some of the higher extensions are more commonly used than others. The most common are the use of the b9 and/or #9 in chord V and the upper extensions of chord I.