.STEP/.F3D to .SCAD file?
Even though OpenSCAD can import a variety of formats, the file structure will not be accepted by Thingiverse in the manner presented by the OP.

OpenSCAD is a text-based description language. One creates parameters assigned to specific aspects of a model and implements those parameters to create the desired shapes/components by typing in a text editor. The native editor for OpenSCAD provides for some management, but notepad or equivalent would work just as well.

The file format of OpenSCAD is text. None of the CAD type modeling programs will provide equivalent text output.

For your objective, you'd have to learn the basics of OpenSCAD (not particularly difficult) and reference the parameters in the STEP files, then assign them to the appropriate labels in OpenSCAD.

If you have particularly complex designs, it can be a handful. It can also be rewarding when you change a parameter as a test and the complete model follows as expected.