For my READY NAS Can Attached external disk be configured to Be a RAID 5 Volume

For ReadyNAS that support port expansion on the eSATA port, you can use a multi-drive chassis that supports that -- it will look like an EDA500.  But, frankly, the slowness of multi-drive transfer over eSATA with port expansion caused me all kinds of issues.

You can also use a USB drive chassis that has it's own internal RAID capability.  You won't be able to monitor the RAID per se via the NAS (unless the manufacturer provides a tool for such that works on a headless Linux system, and I know of none that do), but it will look like a single drive to the NAS with the space from the RAID.

I recommend you avoid a Drobo DAS for this.  Their method of allowing expandability causes the reported available space to be what it could be with expansion, not what's really there.  Without their tool to monitor it (useful only when the unit is directly connected to a PC), you'll be in the dark as to real available space.

If you had two drives in a single USB chassis that show up separately, you might be able to create a RAID on them via ssh, but I don't know what would happen if it were disconnected and re-connected.  Making a RAID from two totally independent USB devices is just too risky.