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.