Make a new mission_template, change the indices under #multiplayer game type in header_commons, change the string references for multiplayer_game_type_names_begin, and modify certain checks (which affect options, scoring, objectives, etc.) in prsnt_game_multiplayer_admin_panel to include your new mode. This also includes script_game_get_multiplayer_server_option_for_mission_template, script_game_multiplayer_server_option_for_mission_template_to_string, script_multiplayer_fill_map_game_types, script_game_multiplayer_get_game_type_mission_template, and probably a bunch of other places. Don't take my word for it though.