The only way that EK will be able to add on the JFK-DCA segement onto the same PNR is by cancelling the ticket you have now and reissuing it. Usually XXX-DXB-JFK tickets are the same price as XXX-DXB-JFK-WAS/BOS tickets so it shouldn't cost you too much more.
If you buy a separate ticket through EK, and it is on a separate PNR they won't be liable to protect you in the case of a delay.
cheers