As a NAS is an actual computer that shares files over a network, thus being called a Network Attached Storage, I don't think you'll ever find one that can be connected to a computer via USB. However, most newer NAS units have a USB port to allow for expansion. So, you could have a NAS (such as QNAP or Synology) and then move a USB hard drive back and forth. But, you may need to re-share the USB drive each time you reconnect it to the NAS.
The other option is to simply use the computer where the USB drive is connected and share it via the OS.
What exactly are you trying to achieve?