In ADS, You can take a model of transistor (without any parameters). Then you can enter the values manually. It will work. I have not done this thing for RF transistors.

However i have tried this thing with RF schottky diodes. 

I agree with Chandra. To make it easy for you, please follow these steps:

1) you can take a copy of current model related to your technology,

2) Then add your equation or new parameter value,

3) Then make sure the extension is ".SCS",

4) Save the file and add it as one of the model files in ADE

Dear Ahmed,

Can you please elaborate your answer?

Thanks

can't we install it for ads 2008 0r 2016 version?