At 4NEC2 plots of STL VSWR I explained a method of working around a limitation of 4NEC2 values for Zo that can be applied using the Settings menu.
I asked the developer to consider a change, but I gathered that he regarded 4NEC2 to be at End Of Life.
It appears that 4NEC2 enforces a requirement that Zo>=0.1, so having discovered that by trial and error, one wondered if it was possible to change that threshold by hacking the exe file.
The IEEE754 Double representation of 0.1 is 0x3FB999999999999A, and of course it would be stored backwords in the exe file. Searching for 0x9A99999999999FB3F found only one occurrence, offset 0x1490. That was changed to 0x8DEDB5A0F7C6B03E (the backwords representation of 0.000001) and the exe tested. (It might be tempting to set it so zero, but that would permit entering zero which may cause run time errors).
To my delight, it now permits directly changing Zo down to 1µΩ.