Once you get past the interview stage, virtually every school (at least that I know of) accepts additional letters. However, if you're in the pre-interview stage, there are some that don't accept update letters.
From my experience when I applied, the schools that didn't accept additional letters were Stanford, UC Davis, and USC Keck... and the schools that did accept letters were those already mentioned plus Dartmouth and GWU.
Yes: Cornell, EVMS, Jefferson, UVa, VCU, Temple, Mayo, UPitt, Dartmouth, GWU
No: Vermont, Stanford, UC Davis, USC Keck