USB-C, with the right cable, connector and machine, can support Thunderbolt which is PCI-Express. If you go for a Thunderbolt adapter (and not a USB-based one, as above), you will get around this problem. You can also get one of those external GPU cases and put a desktop-grade PCIe network card in there and it should work provided your computer has the drivers for it (for Mac, I'd suggest trying an Intel card and hoping for the best).