Unfortunately, like with most over operating systems, adding a 2TB drive to the computer will simply cause another 2TB of separate space to appear as usable on the system. And since in StartOS all the data is on one drive partition, there would be no way to assign this new space to services on that different separate partition. There’s also no way to merge the two drives and have the system act like there are one. Adding a separate 2TB, in StartOS, would simple cause a 2TB backup target to appear.
If you have available network storage and the technical chops, you can host Jellyfin content there, following this guide: