I couldn't find a way to avoid re-importing. The best work-around I found relies on the fact that Evernote allows you to sort by date created and/or updated. If you set up your import folder to the new import, it will re-import the existing records, but the new records will be created/updated on the date the import was set up. Then you can delete all the newly created files (since they're already in your folder) and it will continue to import any files added to the folder. Hope this helps.