That link lays out the process of converting an STL mesh to a workable STEP file. The STEP file WILL be massive and will make your computer chug along like a fat kid on a mountain trail, but it will work. It also touches on the mesh functionality built into FreeCAD for repairing and manipulating existing STL files.
Keep in mind, this does not produce a good Solid part after conversion. It will look like an STL, but will be manipulable with regular CAD software rather than a Mesh Editor. You'll need to figure out the modifications required to convert the raw conversion into a decent part. This will involve determining flat faces to be cut into slightly and then re-extruded to a unified face, radiused corners that need to be redone into a smooth face, and all the other things that need to happen so you end up with a file that isn't 200+ MB. You'll end up recreating the part by hand, but this conversion will give you the advantage of having a starting point, rather than trying to do it all manually.
A tutorial on this cleanup process will be posted later and linked here.