Docker Transmission

You might want to try other torrent programs and see if they are more flexible with the storage locations. rutorrent which I use on seedboxes looks like it could do what you want. Have a look at deluge also.

you need to forward the port used by transmission (51413 by default or whatever else you set it during container deployment) in your router's setting.

first you need to make sure to set up a fixed internal ip address to the device transmission is running on (most likely this will be OMV IP address) then forward the external ports to this IP address. google your router's brand + port forwarding for a detailed howto