I love this generator.  I wanted to use it for cities in a dwarven empire I  am  building, and I   wondered  if  it  would  be  possible  to  add  an  underground  or  subterranean      option  for  city creations?

Hello there, I would suggest to adjust the colorsettings to emulate the underground. Since Underground-Cities may have multiple Levels, it would be hard to visualize in 2 top-Down Perspektive. I use to ink all buildings roofs with black color and changed surounded environment with eartlike colors. I disabled farms and trees, and configured only one city. I always use to finish my City maps with photoshop (or gimp) and add some details, like stairs between leveles, change castlewards etc. 

It's not an uncommon request, but apparently there is no general opinion on how dwarven cities are supposed to look. Some people imagine them as megadungeons, others - as more or less regular settlements with streets and buildings, but "styled" in a certain way (see zombienoxx' comment). Sometimes it's something entirely different...