In CalculiX, the only elements with edge geometry are beam/truss elements, but they aren’t supported in PrePoMax so you can’t import STEP files with just the edges. To use S8R (3D shell) elements, you would have to import surface geometries (usually extracted from solid parts as their midsurfaces).
But be very careful with shells since they have several issues/limitations in CalculiX - symmetry shouldn’t be applied in a standard way (needs local CSYS to handle the drilling DOF) and rigid body constraints cause nonconvergence if Nlgeom is enabled.