There is no MXI to USB adapter that will work (that I know of).
The PXI hardware is basically an extension of the PCIe bus and needs to communicate across said PCIe bus. It also needs to be recognized by the computer at boot.
If you had a different chassis and thunderbolt ports on the computer, you could use the PXIe-8301 which provides communication over Thunderbolt. With the PXIe-1073 chassis, though, your options are limited to either the PCIe-8361 or PCIe-8362. If you have an Expresscard slot, we used to sell an 8360Expresscard, but these are EOL now (making way for Thunderbolt connections).