You can't, but you also shouldn't need to.
If the new master code has a different game ID than the original, then MCM will treat it as a completely separate game, so there won't be any problems adding it.
If it has the same game ID as an existing game, then it will object if you're trying to add it as a new game. In this case, what you should do is to add the existing game to your custom codelist (thereby importing all of its normal codes) and then change the master code afterwards.
If the original game that it's conflicting with isn't the same game, then you should contact whoever you got the codes from and get them to issue new codes without conflicting IDs.