I have been using ADS for my patch antenna design but I now want to simulate
these designs in HFSS.
Do I have to start all the designs from scratch or is there any way to import these designs into HFSS?
Send your ADS design to the Momentum Layout and save it as a DXF file(or
what ever file format you can easily import to HFSS). Now start HFSS and look for
that Momentum file. It should be straight forward. In Momentum go:
"File"-->"Export"
and pick the best format.