Spice

I've made a behavioral simulation in Simulink, now I want to make my design at the circuit level. I was wondering is it possible to model an AMS IC without using library of any specific process in spice ? I've heard that I can give values to the CMOS's parameters to define the I-V characteristics myself. How can I set this parameters when I don't have libraries of a specific vendor ? How valid are the results ?

Many Thanks