this is very easy. all you need to do is mod the descr_sm_factions.txt in your data folder. to start look up a faction you want to change cultures. I'll start with Thrace.
Code:
faction						thrace
culture						greek
symbol						models_strat/symbol_thrace.CAS
rebel_symbol				models_strat/symbol_greek_rebel.CAS
primary_colour				red 161, green 199, blue 199
secondary_colour			red 76, green 76, blue 76
loading_logo				loading_screen/symbols/symbol128_thrace.tga
standard_index				19
logo_index					249
small_logo_index			262
triumph_value				5
intro_movie					fmv/intros/greek_intro_640x480_bars.wmv
victory_movie				fmv/victory/greek_outro_320x240.wmv
defeat_movie				fmv/lose/illyria_eliminated.wmv
death_movie					fmv/death/death_thrace_sand_320x240.wmv
custom_battle_availability	yes
can_sap						yes
prefers_naval_invasions		no
all you have to do is change the word greek to any culture you want.
RTW 1.5 cultures:
roman,barbarian,greek,eastern,carthaginian and egyptian.

BI 1.6 cultures:
roman,nomad,hun,barbarian,eastern,carthaginian (such as African factions)
personally i choose to change the Goths to barbarian. but they can still use the horde feature.

Other things you can mod in the descr_sm_factions.txt:
Code:
can_sap						yes
if you want this faction to have sapping as an assault tool, put yes. most barbarians dont have sap on.
Code:
prefers_naval_invasions		no
if you want this faction to use naval invasion more often, put yes.
Code:
custom_battle_availability	yes
if you want this faction available in custom battles (and quick battles) put it at yes. so you can even play rebels in custom battles (and quick battles)

i hope this tutorial was easy for you!