interesting, nethserver-transmission is not aware I think about VPN, probably if you use globally a VPN for the server then Transmission could use a VPN to download/upload torrent. In France it is mostly when you upload a torrent that you could have problems, even of course if the download of a torrent is forbidden.
However you could use some transmission docker container customized for the VPN, it should be something funny : https://github.com/haugene/docker-transmission-openvpn
I have not really the time yet, but in short I would be interesting by a howto
install nethserver-docker
create the container transmission
create a reverse proxy to use transmission on the container port