It'd be damned hard without the source code to the engine.
I don't know why they left it out. They had the source code for the original Fallout and for Fallout 2, so it's not like it would have taken more than cutting, pasting, and a little editting.
There was a huge outcry from a lot of people to add a dialog tree in to Fallout Tactics when it was revealed it wouldn't have the dialog tree, but Chris Taylor said something like, "It's not that kind of game."
Personally, I wish it were in there also. I wouldn't mind someone making the game more RPGish, but unless they add a dialog tree with the ability to define the dialogs in an editor..