You can import the entire folder structure from your DMS - to do so go to a folder in the Media Pool, click the blue "Actions" button and select "Import". In the "DMS" area you can select the folder(s) that you want to import. If activated, the "Check" option checks for existing files in the entire Media Pool in order to avoid duplicates.
You can import DMS folders into the backend Media Pool as well as into a group Media Pool.