i'll be eagerly following this thread since i'm thinking of incorporating the underdark into my own campaign in the near future. While City Designer doesn't having that sort of thing, I know dungeon designer (and other products from Profantasy) do have stalagtites and other cave oriented objects. You should check out the Dunjinni Forums to find some stuff that could work.

I don't know how to actually create objects myself, but I wouldn't mind learning or helping out if anyone does have any ideas.