Forms: Include a path to a folder within Google Drive

While I suspect it is technically possible via Google's API to do what you want, I suspect SS choose not to add that level of complexity to the form Storage options.

I don't have a solution but it may be possible to write some scripts Google side to take the lines that are coming into the top level document and move them into another spreadsheet that is in a folder.

You of course end up having two forms but if your goal is to use folders for organizational purposes, then this potential technique would get you there.