I don't know specifically for McMurdo, but
rule of thumb is that MMSI can only be programmed by the
equipment vendor/dealer (or if by consumer only once, eg for an
AIS transponder or
VHF DSC). Authorities apparently don't want users to have the ability to change the ID at will (or maybe they just worry about wrong entries).
So you should contact McMurdo customer
service plus any local dealer to see if either can reprogram for you.