Go to world_names file and look at this line:
_CULTURE_EXTRA_0_;User Defined;;;;;;;;;;x
Now when you see it, replace User Defined with your culture name. For example:
_CULTURE_EXTRA_0_;Nethrosian;;;;;;;;;;x
If you want to add this to pop files, go to pop file where you want to add it (expl:ABU1836_AbuDhabi)
You can see the these lines:
1881;Arabic;Sunni;soldiers;;;;2
1881;Arabic;Sunni;farmers;;;;28
1882;Arabic;Sunni;soldiers;;;;2
1882;Arabic;Sunni;farmers;;;;28
1883;Arabic;Sunni;soldiers;;;;2
1883;Arabic;Sunni;clergymen;;;;2
1883;Arabic;Sunni;aristocrats;;;;1
1883;Arabic;Sunni;farmers;;;;25
To add your own in, make a new line:
1882;CULTURE_EXTRA_0;Sunni;farmers;;;;1
This will add 4000 nethrosians to province wich has id 1882
If youre using Notepad etc. make sure that there is only one empty space on the bottom of that file, otherwise it will error.
BTW:I am terribly sorry bout my engrish. Honestly, I am ashamed.