Simulate input resistance
One way would be to use the voltage source and put a resistor in series with it. Then use a parameter sweep on the resistor to vary it's resistance.

The method for doing this is described in detail here:http://www.ni.com/tutorial/12777/en/

In short you can do this from the analyze parameters screen, select the resistor and input different values that it should change to. Multisim will then run multiple simulations and only change the resistor value. You can also run print statements to find things like the output voltage.