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.
Thanks for a good advice. I have a system with 4770K and ZFS and it can handle it fine for Gigabit Ethernet (I don’t need more), so I believe 6700T should do. What’s the purpose of LSI storage adapter, I haven’t heard of this device yet?