STEP and IGES objects are "dumb" solids and surfaces. They give you shape information, but not the means/recipe that they're created by. They were designed to allow some interchange between programs without using proprietary information, because CAD's a competitive industry. There's been an age-old chase and catchup between Autodesk's .dwg and what used to be called the OpenDWG consortium, that grew out of IntelliCAD in 1998 using .dwg as its native format, and didn't Autodesk howl about that? There are some direct editing tools in TC that can be used to rebuild some parts from the external faces of an object. Facet editor lets you isolate faces using "create surfaces from faces", Quick Pull and recently Tweak Face give some means to thicken/solidify them. Depending on their relationship and how you thicken them, it can be possible to intersect those solids to recreate a part, and sometimes if the surface is a shape like the half-cylinder shown in this thread that you can see only needs a flat surface to enclose the volume, then selecting it and invoking "create solid from surface" will do exactly that.