You will also need IDU for what you want to do, as it involves dialogue scripts, as well as TBG Mass Converter to export any .dlg files to .tbg format, which can then be used to modify the existing dialog.tlk file in the game. It's a bit complex, mate!
TeamBG's forum will probably be the best place to advise how to do what you want to do. I'm not sure if you can ump back and forth between the games but Ken, Todd or Cat would be the best people to answer this.
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