yes a disk shelf and another SAS expander could be an good option to get bays if you want to move the freenas instance to a VM with the HBA passed through. Hardware is up to it, assuming you have rack space for another box.
Or just upgrade the disks in situ to higher capacities. Hard to do for $400 though unless you had budgeted separately for disks. How are your pools split across 13 X 4TiB drives?
Those are nice servers but if you want to grow capacity I’d either add a dumb shelf or go with the new 4U server and converge the lot… I’m thinking of doing this in my next upgrade.