Just script the Export command using RhinoApp.RunScript. Here is an example of exporting to DXF. This should give you an idea how to do the same with STEP.
var doc = Rhino.RhinoDoc.Create(null);
var tmpObj = doc.Objects.Add(myGeo);
// TODO: Manipulate tmpObj in the new document, without altering it in the original user document. E.g. reorient it.
// TODO: How do I choose which doc RunScript targets?
var script = $"_-Export \"{filePath}\" _Enter";
RhinoApp.RunScript(script, false);
Maybe this is a convoluted way of thinking about it? Should I instead make a copy of the geo in the current doc, manipulate that, then select it (I find programmatic selection a bit awkward/hard), and then run export selected?