You can try co-design your slot antenna with Ansoft Designer, in which you have to build your own library, and then you can export the Designer schematic into HFSS 3D structure.
Or you can use HFSS-Matlab api, which can be found from google.
But theses tools are not precise and accurate and also take a long time to learn to use and build the lib.
So, I suggest just use the HFSS to draw your antenna. With further operation of rotate, duplicate you can reach a very complicated model.