I was searching for converter of *.stl file into *.dxf and found this online utility:
http://www.cadforum.cz/catalog_en/stl2dwg.asp
It worked OK, and resulting file could be opened in LibreCAD (which is 2D). I have yet to try this with some 3D cad.
Also, there are other utilities that may help editing stl files:
https://all3dp.com/free-stl-editor-open-edit-stl-file/