HFSS will import many CAD formats. If your design is in a format HFSS can't import, looking online will often find you a program which converts to a format HFSS can import.
Beware, however, that HFSS designs using imported files are sometimes much larger (file and memory size) and thus much slower to simulate than designs made from scratch in HFSS. So if possible, it can be better to input the design by hand, or else using the matlab-hfss API (google it, it's very useful).