stl to c4d conversion has been in the past often searched by users that wanted to import STL files to CINEMA 4D, back in the times when the tool did not support STl format. The modern versions of CINEMA 4D actually do support both export and import of STL files, so this should no longer be issue. Of course you will still need CINEMA 4D, considering that its formats are rarely supported in 3rd party apps.
Updated: July 25, 2023