The easiest way to add a model card to the device library is to copy an exiting .model file for a similar transistor . Then determine the model parameters of the intended transistor using the data in the data sheet and a model parameter extractor software. After obtaining the the values of the model parameters substitute it in the copied . model card.
After modifying the values of the parameters you need to recompile the library to activate the new .model card.
About "HSPICE® User Guide: Simulation and Analysis" , Chapter 7 has a very brief descriptions but these are basics for the simulation of created model files, I'm interested in making a library file by myself.
What is your purpose? If you want to simulate an emerging new transistor such as CNTFETs, MoS2FET and etc., you need a Verilog-A model file. Otherwise, for simulating other commercial technology such as BSIM, FinFET, you can find them at the PTM technology model file site.