How to search for documents on OSX that are NOT contained within a given folder using Finder, Spotlight or some other native tool?
You can use HoudahSpot to search for spreadsheets that are outside your centralized folder. Full disclosure: I am the developer of HoudahSpot.

In HoudahSpot you can create a search for spreadsheet files. Specify that you want to search your home folder, but want to exclude the folder you set aside for spreadsheets. Start the search and then drag the search results to the folder where you want to centralize your spreadsheets.

You can also customize the search results list to show additional columns, including the file path.