How to model an equivalent resistor V/I in LTSpice
That is quite possible if I understood you correctly. Just like PlasmaHH pointed out in the comment.

Take this example circuit:



What you like to know is the equivalent resistance of the resistor and transistor, which changes based on the voltage going to base of the transistor.

Now what you easily get is the current waveform by clicking on the resistor:



Of course that is not what you are looking for. As you see I named the net at the top to V66 because this makes it easier to get the correct node for the next step.

In the plot window you can now right click on the I(R1) (or add a new Trace with the menu or Ctrl+A) at the top which gives you an expression editor, where you can write some nifty formulas. An easy one for this problem is: V(V66)/I(R1) which will give you this:



Which is (hopefully) what you want.