Are you talking about an arbitrary 3D (MCAD) HFSS project? I suspect that may be the case here. I don't know of an easy way to get an MCAD design from 3D HFSS to SIwave. If this was an HFSS 3D Layout project it would be pretty trivial as you don't even need to export anything. The .aedb (EDB) database that 3D Layout creates to store geometry inormation can be directly opened in SIwave.