This is done all the time, you just need to have two
MOSFET models fitting the two species and link the
symbol appropriately to the model for each instance
(or, you make a symbol for each FET species that
has a link to the proper model).
If you were going to do this for reals you'd have a
foundry PDK from a flow that offered such options
(two? only two? Seen more, often) and then the
dirty work would already be done for you. Might
drill into the Tanner web site and see what PDKs
they have (whether they will be provided, is your
deal).