We regularly import content from old web sites and systems. One recent client had thousands of documents that we needed to copy from the old site, so we wrote a scraping system to import the ones that fit a certain template into Drupal, and just copy the existing documents into sites/default/files.
Using the Filefield_sources module, you can associate an existing file with a filefield, using IMCE or other files uploaded through the file system. However, we hit a problem: if you try to browse to an existing file, Filefield returns an error when you try to refer to it: