Is Thunderbolt an option? If so, you could probably use one of those external GPU cases and put a proper PCIe HBA in it, to make a TB-to-SAS adapter.
For straight USB, I'm not sure it's possible -- while there are things that convert SAS to USB, such as
this, I'm pretty sure they're just translating the protocol to native USB storage under the hood, and that probably would not work with an expander. You could try emailing them to ask though.