The procedure for editing the .dlg files varies with the editor you are using. Right now, I don't know of any editor that fully support swkotor's VOs fields yet (hopefully Fred will add a dlg editor to his TODO list :) ) For now you can either use the GFF editor, NWN toolset, or the standalone dlg editor for NWN called SpeakEasy (downloadable from
http://www.sourceforge.net/projects/leto) ). As I mentioned, if you're using these latter two editors, you might run into problems keeping the original VO's if you edit the existing dlg files. However, if you make a new dlg files, these editors will work fine.
For dark Bandon, the dialog files that are used when he confronts you are called from a trigger script. So if you want to fight him first, then have him surrender, then the conversation after he surrenders can be in a totally different .dlg file. This means that you can start a fresh .dlg file without editing the original ones.
I'm not sure if I answered your question or not, but feel free to ask again if you need clarifications about anything.