With the STEP file having surfaces that you can sketch on why exactly would you want an editable solid feature tree? I only ask because I think you might be chasing your tail unless you are changing a part that already exists. If you are augmenting with accessories or adapters the STEP is all you need. I made a bracket and a hub for the window motor in Solidworks and have posted them on the Autodesk360 site in .STL format if you're interested.
window motor mount bracket -
http://a360.co/Xg8oGL
window motor drive hub -
http://a360.co/Xg8spU
If I can help in any way let me know!