You can do it with the baseCustomMission setting. Example:
alienRaces:
- id: STR_ANTHROPOD
baseCustomMission: STR_SUBVERTED_HEADQUARTERS
- id: STR_SPITTER
baseCustomMission: STR_SUBVERTED_HEADQUARTERS
- id: STR_PSIMORPH
baseCustomMission: STR_SUBVERTED_HEADQUARTERS
- id: STR_SKELETOR
baseCustomMission: STR_SUBVERTED_HEADQUARTERS
- id: STR_MICRONOID
baseCustomMission: STR_SUBVERTED_HEADQUARTERS
As an addition to what Meridian said, you can also put random members into a race. Example:
alienRaces:
- id: STR_EARLY_INTRUSION
members:
- STR_PSIMORPH
- STR_CHRYSALIS
- STR_CHRYSALIS
- STR_ALIEN_EGG
- STR_ANTHROPOD
- STR_SPITTER
- STR_POPPER
- STR_POPPER
membersRandom:
- [STR_PSIMORPH,STR_PSIMORPH,STR_REAVER]
- [STR_CHRYSALIS,STR_ALIEN_EGG]
- [STR_CHRYSALIS]
- [STR_ALIEN_EGG]
- [STR_ANTHROPOD]
- [STR_SPITTER,STR_SPITTER,STR_ANTHROPOD]
- [STR_POPPER]
- [STR_POPPER,STR_BRAINSUCKER]
You can also use membersRandom to make rare encounters:
membersRandom:
- [STR_TURTLE,STR_TURTLE,STR_TURTLE,STR_TURTLE,STR_TURTLE,STR_TURTLE,STR_TURTLE,STR_TURTLE,STR_TURTLE,STR_GIL_TURTLE]
- [STR_GOBLIN,STR_GOBLIN,STR_GOBLIN,STR_GOBLIN,STR_GOBLIN,STR_GOBLIN,STR_GOBLIN,STR_GOBLIN,STR_GOBLIN,STR_TREASURE_GOBLIN]
- [STR_DUCK,STR_DUCK,STR_DUCK,STR_DUCK,STR_DUCK,STR_DUCK,STR_DUCK,STR_DUCK,STR_DUCK,STR_GOOSE]
- [STR_CACTUS,STR_CACTUS,STR_CACTUS,STR_CACTUS,STR_CACTUS,STR_CACTUS,STR_CACTUS,STR_CACTUS,STR_CACTUS,STR_CACTUAR]
- [STR_BLUE_DRAGON,STR_BLUE_DRAGON,STR_BLUE_DRAGON,STR_BLUE_DRAGON,STR_RED_DRAGON,STR_RED_DRAGON,STR_RED_DRAGON,STR_HEXADRAGON_DRAGON,STR_HEXADRAGON_DRAGON,STR_MALBORO]
- [STR_EXCALIPOOR,STR_EXCALIPOOR,STR_EXCALIPOOR,STR_EXCALIPOOR,STR_EXCALIBUR,STR_EXCALIBUR,STR_EXCALIBUR,STR_MASAMUNE,STR_MASAMUNE,STR_ZANMATO]
- [STR_MUTON_SOLDIER,STR_MUTON_SOLDIER,STR_MUTON_SOLDIER,STR_MUTON_SOLDIER,STR_MUTON_SOLDIER,STR_MUTON_SOLDIER,STR_MUTON_SOLDIER,STR_MUTON_SOLDIER,STR_MUTON_SOLDIER,STR_MUTON_COMMANDER]
- [STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET,STR_LOSING_LOTTERY_TICKET]