Re-using existing volume with docker compose

Per the documentation, using the external flag allows you to use volumes created outside the scope of the docker-compose file.

However, it is advisable to create a fresh volume via the docker-compose file and copy the existing data from the old volumes to the new volumes

Yes you can do it normally, just an example below: Set external to true and set name to the name of the volume you want to mount.

version: "3.5"
services:
  transmission:
    image: linuxserver/transmission
    container_name: transmission
    volumes:
      - transmission-config:/config
      - /path/to/downloads:/downloads
    ports:
      - 51413:51413
      - 51413:51413/udp
    networks:
      - rede
    restart: always

networks:
  rede:
    external: true
    name: rede
volumes:
  transmission-config:
    external: true
    name: transmission-config