Even with DXB, it's likely an invalid routing. My guess is Emirates doesn't have a Washington to Athens rate. You'd be overflying Athens by over 2000 miles, meaning youre adding 4000 miles to the trip from Washington to Athens -- which is only 5000 miles total.
If you really want to do this, buy two separate tickets. It will price as a WAS-DXB flight and a DXB-ATH flights.