> Descent 1&2 (the game) enemies: the list of enemies appears at several places in the codebase, so you can't simply add one (=the density of interconnections is too high).

I'm not sure why you picked such a specific example, but it looks like read_robot_info() will happily read in additional robots from descent2.ham if you increase MAX_ROBOT_TYPES and stick them in the same global. There might be specific behavior info sprinkled elsewhere in the game, but that is probably code you'd need to touch to implement the uniqueness of new enemies anyway.