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.