Not having used QUCS, this will be a very generic answer.
It looks like QUCS, in addition to having a standard MOSFET template (looks like Shichman-Hodges), also supports Verilog-A. First thing to check is if the ADS model from Avago can be ported or converted to Verilog-A. If not, you will need to extract parameters from either the datasheet or measurements to map into either:
- QUCS standard model (Shichman-Hodges)
- Roll your own Verilog-A lumped physical model
- Roll your own Verilog-A behavioral equation based model
The QUCS site has some papers describing creation of Verilog FET models, like: "Verilog-A compact device models for GaAs MESFETs"
It's not clear how complete the model needs to be, and what it needs to include. You might want to modularize the model. Put the device into one model, but all the package parameters into another model that the device can be plugged into later. That way the package part might be more generic.
Remember that all models are wrong. The only accurate model is the thing itself, and there is not much advantage in that. So, try to make whatever model you use as numerically light as possible, while still being accurate enough to be useful in the region of interest. Keeping things focused on a region of interest will help the model from becoming so numerically ponderous that it fails to converge for any practical circuits.
Excuse me if I've just stated what is too obvious here. Good luck.