TrueNAS has a feature built in called Jails that allow you to run applications in their own walled area similar to a VM but more similar to a Container. There used to be a section (haven’t used TrueNAS in a moment) where you could easily attain official and community pre-built jails and install them without much difficulty. QBittorrent was in one of those lists. Definitely worth poking around to see if that’s still there.
Another option would be to stand up your own jail and install the program you want on there yourself. Here’s a quick rundown out there that seems to cover the basics you are looking for.