You can add radarr, etc. to your existing stack without changing much. You mount your movies directory to /mnt/movies in the radarr container, shows to /mnt/tv in sonarr, and so on. And the qbittorrent downloads folder to /mnt/downloads. Then, when you add a new movie to radarr, it’ll add it to qbittorrent and get downloaded to the downloads folder, and when it’s done, radarr will move it from there to the movies folder. Then jellyfin sees the new file and adds it.

If you want to do everything the “radarr way”, you’ll also want to import the existing library to radarr so that it can identify the movie quality. There are buttons in radarr, etc. to rename and move files the way radarr wants them. Since nothing else cares about this, I let radarr handle the naming. During first setup there was a lot of re-downloading of media it couldn’t identify the quality of, but I didn’t care about that so I let it happen. But all of this is optional, you can disable quality upgrades and leave your existing library alone.