No direct exporter, for deep mathematical reasons.
SU works with lines (vertex) that then create faces, some of them creating solids.
Stem file is a nurb file.
Your best guess is to go through a third party, fusion360 apparently reads SU files and exports step files, and there is a free version for hobbyists. the threads mentions Punch2D/3D, Rhino, Freecad (using a STL)
It’s a similar issue with many other softwares, autocad, revit, archicad, they all need to use a multi step solution or a workaround to translate to step.