What's the unclear point of "not modelled"? What do you want to achieve?
You can e.g. make your own transistor level or behavioral model. It will most likely not represent the device behaviour very well, but at least model the offset pins.
It depends on the models. Some implement an arbitrary offset voltage as "reality" factor, others are ideal in this regard.
In any case it's really easy in a simulation to add a variable offset respectively offset adjustment voltage to the circuit. In so far I don't yet understand the problem behind the question.
I looked at a bunch of PSpice models formOA's with offset adjust pins and found none in the model. Then I read on one site they are never supplied unless a high precision chip.