The CDF parameters will end up as instance parameters, so you'd have to arrange that you have some kind of subckt model for your transistor where you can pass in the model parameters. In other words, your nfet.scs would look something like:
inline subckt nfet (d g s b)
parameters w=1u l=1u tox=1e-7 vto=0.6
model nfet mos3 type=n tox=tox vto=vto // and any other parameters you want to be fixed
nfet (d g s b) nfet w=w l=l
In the above, I've used a mos3 model, but of course you'd define whatever model you want (bsim4, bsim3v3, etc, with the appropriate parameters). Because the model is parameterized, and defined within the subckt, it can use any parameters passed to an instrance of nfet.
Then you'd define these additional parameters as CDF parameters in the CDF for the device, and list them in the instParameters in the spectre simInfo so that they get netlisted.
Does that help? Sorry it's a bit brief...