How can I demonstrate magnetic and capacitive coupling on an STP cable in SPICE?
You can add interfering magnetic coupling to each line by using three inductors and a coupler called "k". Two inductors are placed in series with each line and the third is driven by an independant source of the noise or interference. "k" is a component in all spice tools and it defines the coupling factor (0 to 1) between the driven inductor and the two series inductors. That is how I have done noise coupling in the past.

Inductor values should be tens of nano henry to hundreds of nano henry typically for this type of simulation.

Capacitive coupling requires a capacitor from each line to a driven point. The driven point is a voltage source of interference to ground.

As with both of the above if the impedance in each line (or connected to each line) is slightly different you get a differential coupling as well as a common mode coupling.