@MK83- sorry for long delay replying, it's very hectic at the moment, not much time to breath. Anyway, back to your question. No, unfortunately, there is no possible way to do underground stuff. I think GG would need some sort of CSG cutting ability that allows you do tunnelling. That would be quite a major dev task.
On another note regarding HM2GG app, I have reworked/converted C code (taken from Gimp normalmap plugin) that will recreate/convert Normal maps to Heightmaps. It's a about 99.9% there. See below pics from a sample normal map. This is good if you no longer have or never had the original height map. It's still an ongoing experiment, playing around with and swapping various values and also altering contrast level etc. As free time is becoming less and less, I hope to release a major revamped version of HM2GG within the next few weeks.
Hardware: Dell Precision 490; AMD Radeon HD 7570; 12GB.