I don't think it would work to add segments. E-tickets are made up of "coupons" for each segment you are ticketed on. It requires a reissue to change the PHL-BWI coupon to PHL-PIT-IAD, since you need two "coupons" to do PHL-PIT-IAD.

If you want to standby on a PHL-IAD nonstop, it should be no problem, just ask when you check-in at SFO, but it seems like you want more segments, which is basically impossible to do unless you are involuntarily rerouted.

Even though fare rules don't always allow it, IAD/DCA/BWI are co-terminal and therefore you can try to standby to one with a ticket to the other.