I’d start over. A mobo, decent CPU, lots of ram for ZFS, and a used LSI storage adapter from eBay would get you far on a budget. USB won’t have the performance or reliability and you’d need a handful of drives to make it worth it. You could install an M2 SATA adapter, I don’t know if it would even work, and drive drives with it but they would still need power and you couldn’t put the case back together without cutting holes in it.
Disk redundancy. Let’s you plug in many drives. I have eight 16TB drives in a raid z2. I can lose some drives and not lose data.