IGES and STEP are neutral formats for sharing splines and surfaces, not meshes. If you want to convert your mesh to something you can export as IGES or STEP, you'll need to surface it first. The surface is not the same as the mesh; it will usually have less detail but be smoother. It's a lot easier to go the other direction; meshing the surfaces to create something that can be exported as STL, PLY, or OBJ.