We use the SFTP module for this. But the file names should contain something you can use to retrieve the correct user. This way you can retrieve all those document from a SFTP server (if the client has none we use our own NAS for this). After retrieval we proces the files and mark those where no user can be found.
We made it even fancy so I can define multiple filename configs so I can not only retrieve the user but when put in the filename with a seperate marker use creation dates, type of documents original filename etc.