Can anyone tell me which files I must alter to make a faction start knowing some lands around them, much like EB did??
There is no such file.
To do something like this you have to:

Add spays to proper faction and place them in right position (where you dont want to have FOW).
Then you add campagne script to campagne.
With this script you kill all surplus spays you placed earlier to lift FOW.
At the end you dismiss all messages so player is not flooded with spay killed Messages at the beginning on campagne.

Good luck.