I couldn't get it to work very well...
I found a file called "entity_default" and just added [INDIV_CONTROLLABLE] above the Goblin entity.
I could play as a goblin, but it wasn't what I was expecting it to be.
you can only play as a "play now" character of course, so you have no skills. Its best to make a nice kickass character if you want to so that. Like my dwarven wargod, with poisonous stabbing tentacles and impenetrable steel skin.
Whoa...wait...
How did you get a dwarf with stabby tentacles and steel skin?
YOU MUST TELL ME!
please...
[CREATURE:WAR_GOD]
[NAME:War God:War Gods:War God]
[TILE:'M'][COLOR:5:0:1]
[MODVALUE:5]
[EXTRACT:Plague Spores:7:0:1]
[EXTRACT_VALUE:100]
[EXTRACT_PARALYZE]
[EXTRACT_ANTIDOTE:giant desert scorpion
[liKES_FIGHTING]
[GENPOWER:5]
[CANOPENDOORS]
[NOT_BUTCHERABLE]
[NOPAIN][EXTRAVISION][NOBREATHE][NOBLEED][NOSTUN][NONAUSEA][NOEMOTION][INTELLIGENT]
[NOSTUCKINS][SEVERONBREAKS][NOSKULL][NOSKIN][NOBONES][NOMEAT][NOTHOUGHT][NOEXERT]
[BUILDINGDESTROYER:2]
[NOFEAR]
[PREFSTRING:stern appearance]
[NOSMELLYROT]
[BODY:HUMANOID_SIMPLE:SIX_TENTACLES]
[SIZE:20]
[ATTACK:MAIN:BYTYPE:GRASP:Jab:Jabs:10:15:PIERCE][SPECIALATTACK_INJECT_EXTRACT:50:100][ATTACKFLAG_WITH]
[DAMBLOCK:9000]
[NO_DRINK][NO_EAT][NO_SLEEP]
[ALL_ACTIVE]
[NO_GENDER]
[FIXED_TEMP:25000]
[MATERIAL:METAL:IRON]
[SWIMS_INNATE][SWIM_SPEED:2500]
use that and you'll be nigh invincible