Any advice on how to combine them into a single file,
although I dont have the S12, S21, S31 etc.
I mean S-parameters are a long touchstone file list for given freqeuncies
but the generic mixer device in ADS has these.
I dont know what they mean.
I can not understand what you want to mean at all.
Maybe no one can understand it.
However use DAC(Data Access Component).
but I am not sure what type of simulation I should run to get correct results at output.
If you truely require baseband signal processing, there is no option except for Transient Analysis.
And you can not use system model prepaired in ADS, since they are frequency domain models.
You have to build your own custom models based on time domain descriptions.
I should say, verilog-a or fdd are not things that I know.
Learn them.
Many system models prepaired in ADS are written by FDD.
SDD and FDD are behavioral modeling ability of ADSsim native.
On the other hand, Verilog-A is generic and is not restricted to ADSsim.
However you can not utilize full ability of ADSsim if you use Verilog-A.
SDD and Verilog-A are time domain modeling.
FDD is frequency domain modeling.