I do not know much of ngSpice (yet), but it seems logical to me that it would be possible to define some mathematical formula to add a new variable in a text box in KiCad and pass that along to ngSpice (just like the “.ac dec 20 10 10Meg” ) and ngspice adds it to it’s netlist. You would have to read the ngSpice manual to dertermine if this is possible and how to handle it.
Edit / Oops:
This probably would not work. See:
https://forum.kicad.info/t/how-to-improve-the-kicad-ngspice-interface/15880/12?u=paulvdh