ADC error SPICE simulation
The way to do this would be to put noise sources into the simulation, you don't need to put them all in, just the largest sources (so calculate what they will be and then add them in)

You can add in a noise source by using a voltage source and in SImetrix you can change a voltage source to be a noise source. Either calculate the amplitude and then use that for the amplitude of the voltage noise source.


  noise interval rms_value [start_time [stop_time]]
  
  Source generates a random value at interval with distribution such
  that spectrum of signal generated is approximately flat up to
  frequency equal to 1/(2*interval). Amplitude of noise is rms_value
  volts. start_time and stop_time provide a means of specifying a time
  window over which the source is enabled. Outside this time window, the
  source will be zero. If stop_time is omitted or zero a value of
  infinity will be assumed.
  Source: SImetrix voltage source



Source: https://www.planetanalog.com/author.asp?section_id=483&amp;doc_id=564068

Unless your doing really low level measurements (uV to nV) the resistors will proabably not make much of a difference. I would calculate the Input Refereed Noise and then put a voltage source on the positive terminal (you could also put a current noise source between terminals, but usually the input refereed noise is the largest noise source).

Remember that noise sources add by the sum of the squares, and you are also not simulating 1/f noise (which is hard to do).

Edit: 
Another way to do this with a Monte Carlo simulation is test the edge cases (which would be -5mV and 5mV). So if you know that the total error (or really in this case the DC offset of the amplifier) is max 5mV (on the input) then set the voltage source on the positive input to 5mV and run the monte carlo simulation, then do the same for -5mV. This way you only have to change 1 value of a parameter.