Go to your Dwarf Fortress folder, and click on the sub-folder titled 'raw'. Then click on the sub-folder titled 'objects'. You will see a large list of documents. You'll want to click on the one titled 'entity_default'. Once the document opens, you'll see a massive amount of entries relating to some creature or another. Find the entry for the creature you want by using Ctrl+F (This is the easiest way), and add the tag [ADVENTURE_TIER:4] underneath the creature's entry. If you have already modded a creature into adventure mode, or wish to add another creature in after the first, you'll have to increase the number '4' to a bigger number. Here's an example of what I did:
I wanted to add kobolds in, because they're adorable scamps and I love them. I opened up the 'entity_defualt' file and did a Ctrl+F search for kobolds. Once I found them, it looked like this:
[ENTITY:SKULKING]
[CREATURE:KOBOLD]
[WEAPON:ITEM_WEAPON_SWORD_SHORT]
[WEAPON:ITEM_WEAPON_SPEAR]
[WEAPON:ITEM_WEAPON_BOW]
[AMMO:ITEM_AMMO_ARROWS]
[WEAPON:ITEM_WEAPON_DAGGER_LARGE]
[ARMOR:ITEM_ARMOR_TUNIC:COMMON]
[PANTS:ITEM_PANTS_LOINCLOTH:COMMON]
[INSTRUMENT:ITEM_INSTRUMENT_DRUM]
[CLOTHING]
[SUBTERRANEAN_CLOTHING]
[USE_CAVE_ANIMALS]
[USE_ANIMAL_PRODUCTS]
[FRIENDLY_COLOR:6:0:0]
[DEFAULT_SITE_TYPE:CAVE]
[LIKES_SITE:CAVE]
[TOLERATES_SITE:CAVE]
[START_BIOME:MOUNTAIN]
[START_BIOME:NOT_FREEZING]
[BIOME_SUPPORT:ANY_WETLAND:3]
[BIOME_SUPPORT:ANY_DESERT:1]
[BIOME_SUPPORT:ANY_FOREST:2]
[BIOME_SUPPORT:ANY_LAKE:2]
[BIOME_SUPPORT:MOUNTAIN:3]
[BIOME_SUPPORT:ANY_GRASSLAND:1]
[BIOME_SUPPORT:ANY_SAVANNA:1]
[BIOME_SUPPORT:ANY_SHRUBLAND:1]
[BIOME_SUPPORT:ANY_RIVER:1]
[PROGRESS_TRIGGER_POPULATION:1]
[PROGRESS_TRIGGER_PRODUCTION:1]
[PROGRESS_TRIGGER_TRADE:1]
[PROGRESS_TRIGGER_POP_SIEGE:3]
[PROGRESS_TRIGGER_PROD_SIEGE:0]
[PROGRESS_TRIGGER_TRADE_SIEGE:0]
[ACTIVE_SEASON:SPRING]
[ACTIVE_SEASON:SUMMER]
[ACTIVE_SEASON:AUTUMN]
[ACTIVE_SEASON:WINTER]
[AMBUSHER]
[ITEM_THIEF]
[SKULKING]
[METAL_PREF] -- metal weapons of their own for now
[STONE_PREF]
[MAX_STARTING_CIV_NUMBER:100]
[MAX_POP_NUMBER:2000]
[MAX_SITE_POP_NUMBER:120]
[SCOUT]
[PERMITTED_JOB:MINER]
[PERMITTED_JOB:TRAPPER]
[PERMITTED_JOB:ANIMAL_DISSECTOR]
[PERMITTED_JOB:STONECRAFTER]
[PERMITTED_JOB:BONE_CARVER]
[PERMITTED_JOB:HERBALIST]
[PERMITTED_JOB:FURNACE_OPERATOR] -- gives them metal from ore for the time being
[ETHIC:KILL_ENTITY_MEMBER:PUNISH_EXILE]
[ETHIC:KILL_NEUTRAL:ACCEPTABLE]
[ETHIC:KILL_ENEMY:ACCEPTABLE]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:UNTHINKABLE]
[ETHIC:TORTURE_FOR_INFORMATION:NOT_APPLICABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:UNTHINKABLE]
[ETHIC:TREASON:UNTHINKABLE]
[ETHIC:OATH_BREAKING:NOT_APPLICABLE]
[ETHIC:LYING:NOT_APPLICABLE]
[ETHIC:VANDALISM:NOT_APPLICABLE]
[ETHIC:TRESPASSING:NOT_APPLICABLE]
[ETHIC:THEFT:NOT_APPLICABLE]
[ETHIC:ASSAULT:PERSONAL_MATTER]
[ETHIC:SLAVERY:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_OTHER:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_KILL:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:UNTHINKABLE]
[BANDITRY:10]
[LOCAL_BANDITRY]
Once I had found it, I simply added that tag in, resulting in this:
[ENTITY:SKULKING]
[CREATURE:KOBOLD]
[ADVENTURE_TIER:4]
[WEAPON:ITEM_WEAPON_SWORD_SHORT]
[WEAPON:ITEM_WEAPON_SPEAR]
[WEAPON:ITEM_WEAPON_BOW]
[AMMO:ITEM_AMMO_ARROWS]
[WEAPON:ITEM_WEAPON_DAGGER_LARGE]
[ARMOR:ITEM_ARMOR_TUNIC:COMMON]
[PANTS:ITEM_PANTS_LOINCLOTH:COMMON]
[INSTRUMENT:ITEM_INSTRUMENT_DRUM]
[CLOTHING]
[SUBTERRANEAN_CLOTHING]
[USE_CAVE_ANIMALS]
[USE_ANIMAL_PRODUCTS]
[FRIENDLY_COLOR:6:0:0]
[DEFAULT_SITE_TYPE:CAVE]
[LIKES_SITE:CAVE]
[TOLERATES_SITE:CAVE]
[START_BIOME:MOUNTAIN]
[START_BIOME:NOT_FREEZING]
[BIOME_SUPPORT:ANY_WETLAND:3]
[BIOME_SUPPORT:ANY_DESERT:1]
[BIOME_SUPPORT:ANY_FOREST:2]
[BIOME_SUPPORT:ANY_LAKE:2]
[BIOME_SUPPORT:MOUNTAIN:3]
[BIOME_SUPPORT:ANY_GRASSLAND:1]
[BIOME_SUPPORT:ANY_SAVANNA:1]
[BIOME_SUPPORT:ANY_SHRUBLAND:1]
[BIOME_SUPPORT:ANY_RIVER:1]
[PROGRESS_TRIGGER_POPULATION:1]
[PROGRESS_TRIGGER_PRODUCTION:1]
[PROGRESS_TRIGGER_TRADE:1]
[PROGRESS_TRIGGER_POP_SIEGE:3]
[PROGRESS_TRIGGER_PROD_SIEGE:0]
[PROGRESS_TRIGGER_TRADE_SIEGE:0]
[ACTIVE_SEASON:SPRING]
[ACTIVE_SEASON:SUMMER]
[ACTIVE_SEASON:AUTUMN]
[ACTIVE_SEASON:WINTER]
[AMBUSHER]
[ITEM_THIEF]
[SKULKING]
[METAL_PREF] -- metal weapons of their own for now
[STONE_PREF]
[MAX_STARTING_CIV_NUMBER:100]
[MAX_POP_NUMBER:2000]
[MAX_SITE_POP_NUMBER:120]
[SCOUT]
[PERMITTED_JOB:MINER]
[PERMITTED_JOB:TRAPPER]
[PERMITTED_JOB:ANIMAL_DISSECTOR]
[PERMITTED_JOB:STONECRAFTER]
[PERMITTED_JOB:BONE_CARVER]
[PERMITTED_JOB:HERBALIST]
[PERMITTED_JOB:FURNACE_OPERATOR] -- gives them metal from ore for the time being
[ETHIC:KILL_ENTITY_MEMBER:PUNISH_EXILE]
[ETHIC:KILL_NEUTRAL:ACCEPTABLE]
[ETHIC:KILL_ENEMY:ACCEPTABLE]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:UNTHINKABLE]
[ETHIC:TORTURE_FOR_INFORMATION:NOT_APPLICABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:UNTHINKABLE]
[ETHIC:TREASON:UNTHINKABLE]
[ETHIC:OATH_BREAKING:NOT_APPLICABLE]
[ETHIC:LYING:NOT_APPLICABLE]
[ETHIC:VANDALISM:NOT_APPLICABLE]
[ETHIC:TRESPASSING:NOT_APPLICABLE]
[ETHIC:THEFT:NOT_APPLICABLE]
[ETHIC:ASSAULT:PERSONAL_MATTER]
[ETHIC:SLAVERY:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_OTHER:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_KILL:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:UNTHINKABLE]
[BANDITRY:10]
[LOCAL_BANDITRY]
Notice the adventure tag I added just under the kobold word. See how it's done? That's it.
EDIT: Oh, and I do believe you'll have to generate a brand new world for this to take effect, so don't forget that.