OK, it has been a little while since I looked at this, but this more or less how it works. To start get the kotor tool. (
http://www.starwarsknights.com/tools.php)
To change the appearance of any character into any other character for all instances, you can alter the appropriate entries in appearance.2da. Basically, appearance.2da contains all the appearance data (duh) for every character in the game. Each character has an entry that controls what models and textures they use for various gear if necessary. To give a character the appearance of another character, you modify the entry to indicate that other character's model/texture is to be used. Make sense?
Luckily for us the model and textures for the Dark Jedi in K1 are included in K2. This is where my memory gets a little hazy in terms of what specific entries need to be altered. But it goes something like this...
Entry 379 should be the one for the original K1 Dark Jedi. The Sith Assassins I believe are from 525 to 530. It might even include 531-536. Not sure. In any event, will copy the values from the line at 379 into the sith assassin lines (although the important stuff is the race, racetex, modela, and texta, but I usually just copy the whole thing). Now when the game looks up the Sith assassin appearances, it will find the Dark Jedi appearance.
Give it a try and let us know how it goes. Hope that helps. :)