Which of these do you actually want to do?
1) Add another branch (i.e. a seperate line of research, not connected to the original tech tree)
2) Add to the actual base tech tree (i.e. having tech follow on from the pre-existing techs)
3) Make a whole new tech tree
1 & 2 are actually quite simple.... Mr. DM04 above gave me a link ---->
Link <----
All you really need to do is actually look at the way the techs are structured. If you want to make a new branch - say an Economics tech, it would be something like this.
-Culture ID="Basic Eco Theorem I"-
-DisplayName-Basic Eco Theorem I-/DisplayName-
-Cost-100-/Cost-
-Description-Economic Theorem.-/Description-
-Details-This theorem will enable to make our economy more efficient-/Details-
-EconomicsAbility-2-/EconomicsAbility-
-TradeAbility-2-/TradeAbility-
-SocialProductionAbility-2-/SocialProductionAbility-
-DiplomacyAbility-2-/DiplomacyAbility-
-CultureAbility-2-/CultureAbility-
-Category-Government-/Category-
-Model-stardemoc0-/Model-
-AIValue-10-/AIValue-
-/Culture-
Change the -'s to the < and > & you'll need to look at how the techs are done.
Copy and paste it, using notepad, into the tech tree (GalCiv2, Data, English (or other language) Tech Tree. I suggest you put it at the end, but before the last line. Remember to do the bit above with the tags, otherwise it won't work, or the game will crash. Also, it will only be usable when you start a new game.
If you need any more help, refer to here or the link I gave you.
3, you can't do.
Hope this helps