Yes, you can use ANSYS HFSS to perform RCS simulations. ANSYS HFSS provides several solver methods: FEA frequency domain, MoM Integral Equation, Time domain FEA.
Here's a link to our web-page about RCS, plus here's a HFSS update video that goes into more detail about our RCS capability.
Note that due to the typical applications of RCS simulations (military, ITAR etc), we are unable to provide specific guidance in this public forum.
The study of micro-doppler effects have been gaining interest due to popluarlty of smaller UAV's and the requirement to differentiate them from birds that have similar RCS. A masters student at Chalmers University published their excellent work using ANSYS HFSS for this application in 2017. The paper is published here.
Also, ANSYS HFSS is not integrated into ANSYS Workbench, or the free ANSYS Student product - HFSS is a separate product with a different GUI (Electronics Desktop).
I hope this helps, although my post here comes quite late!