Express Card is one lane (1x) of PCIe (PCI Express) and one USB 2.0 port. The majority of Express Card peripherals I have seen only make use of the USB 2.0 port, with the exception of some drive interfaces and Firewire card, which do make use of the PCIe lane.
Given that, I would think it is *possible*, though perhaps not likely, that you could use an Express Card device with the Amiga via USB.
Long shot, though. :-)