I managed to setup haugene/transmission-openvpn by the help of TechnoDadLife's video.
In this video he says, that other docker containers can use the VPN tunnel of it. I would like to use SabNZBd to use ist. Somebody mentioned in the comments, that it is in the Docker / Networks tab...but I have no clue what to do there.
I'm sorry, all my not-much-at-all-knowledge about OMV I gathered from youtube-videos or tried to replicate from threads in this forum.
I'm thankful for any kind of hints!