How can I calculate complex values in LTSpice?
The complex part will either be a capacitor or an inductor. Just figure out the value for such component starting out with how much reactance you want. And that component will have to be in series with a resistor which is the real part.

Now if you happen to have a negative real part, you need to simulate a negative resistance, which can be done in LTSpice. Click here to see how you could do it if you had a negative real part