Technically speaking you can absolutely purchase a refundable fare BUF-IAD-PLS and drop the BUF-IAD segment close in because you hold the space on IAD-PLS. It will reprice as IAD-PLS, but this is not going to be a significant change in price. However, in your case this is probably more than you want to pay (BUF-PLS in refundable J is something like $2,800 r/t) and in fact you cannot purchase a refundable ticket (at any price) to PLS during the winter holiday period.
I think jsloan's solution is probably best. BUF-IAD is $333 each direction in refundable U class.