I don't think so. I wanted to do just that but found that the USB's on most (if not all) NAS drives are for plugging more stuff into (i.e. another drive/storage device or a printer). I did find one make that claimed to allow ethernet connection as well as USB for read/write but when I dug a bit deeper it turned out it was a partitioned drive with ethernet accessing one and usb accessing the other and it mirrored the partitions to give the impression of access via both methods. But if you do find one let me know!!
That's still the wrong way round. Graham is looking for a drive that can be a NAS on an Ethernet and also a slave device on a USB connection. USB ports on NAS drives are master ports to connect an external drive or stick to.
My best suggestion would be to get a NAS with an extension USB, and a portable drive to swap between the NAS and the Humax. However, if you are happy to run the MSP, I would have thought an external drive attached to the Humax would do all you need without an additional NAS.