Convert .STL file to solid body

It's a matter of how accurate you need it to be. Will you manufacture this model or do you need it for reference in an assembly ?
In this rather simple model, you can cut a number of sections and use for regular features such as extrude , Through Curves and blend.
On a more complex shape you might map/wrap surfaces to the pointcloud.
but, since the .stl file in reality contains only point data, there will be linear splines ( degree 1) between the points and not smooth curves like arcs and ellipses and higher degree splines. - It's easy to be fooled by the smoothly looking display.
1) cut x-y-z sections using datumplanes. ( You can also create sections using the option "Output sampled points" which will create points instead of a un-usable curve)
2) create datumplanes on relevant positions based on the sections created above ( where the shape transitions occur)
3) cut sections using the datumplanes
4) create new curves on top of the splines,( Lines, arcs and splines depending on the shape), or clean the splines into something usable.
5) create features.

The supplied .stl file has some strange scale, when i imported it in millimeter, it gets very small, total length ~25mm, if i import it in inches it gets larger than expected.
Attached is a model ( strange scale) where i have started working. new/ Cleaned curves in green, linear splines in blue.

Regards,
Tomas

Hi Tomas,
thank you for your clear and very useful response, and again for the file attached.
Reading on the web, I learned that Rhino open stl and convert to step.
I will try this solution too.

Thank you...

Using NX 8 and TC9.1