Currently you cannot, the Eeschema/ngspice interface is still limited here.
A hacky solution for such functions:
Make a subcircuit model and use a symbol (here with one input and one output, but may be more inputs, the ngspice B source is very flexible).
.subckt functionplot in out
B1 out 0 V=V(in)/<value of R6>/100
Put this into a file, attach it to the symbol, connect the symbol to the input node(s) of interest, add a global label to the output (if you are just interested in simulation, not the PCB, global labels as node names are o.k.), run the simulation and plot the output of the new symbol.