You can do in two ways:
First method:
Plot all the S parameters (S11,S21,S12,S22) in magnitude and as well as in phase(deg). Right click on the graph and export data in .csv. Copy the data into a notepad file and add this extra line at the top of the data: #GHz S MA R 50
Note the frequency should be in GHz. And the order of S parameters should be
S11(mag), S11(deg),S21(mag),S21(deg), S12(mag),S12(deg),S22(mag),S22(deg)
and save the notepad file as .s2p (touchstone format). And open this file in ADS.
Second Method:
Instead of plotting, create a dataplot under reports in HFSS , in the same order mentioned above and follow the above procedure.
Let me know if you have any problems.
-sv