You can edit classes only to a certain degree. You can create new classes that aren't Jedi so examples of this would be something like a Bounty Hunter and an Assassin. I BELIEVE that you can edit what feats are gained but I can't remember how or where I saw it. It would probably be in either the class.2da, classfeatgain.2da, feats.2da, or featgain.2da files. A lot of the stuff you can look at and kind of guess as to what it means.
But to make a new class is really easy. All you need is KOTOR Tool and the TLKEditor Program. You need to navigate to BIFs/2DA/2da/class.2da. Here you will see a list of classes and it's a simple matter of copying a row that is as similar to your new class as possible. For example if you want an Assassin, using the Scoundrel class as a template is your best bet or if you want a medic, a Scout would work well.
Once you have your new class you must open up the dialog.tlk file in the KOTOR or TSL main directory depending on what game your modding. You'll have to add a new entry for the name of your class for example Assassin by going in under the tools menu. Once you have your new entry copy the entry number off to the left into the appropriate row in the class.2da file, in the new row. I think it's either label or name. Then save the dialog.tlk file and then the 2da file in the override folder.
Now you're ready to apply your new class, the easiest way is to take a companion such as Carth or Atton and open up their utc file. You can find it by going into BIFs/Bluprints/BluprintCharacter/carth.utc, n_carth.utc, or p_carth.utc, I can't remember the name of the file system that the makers used.
Oh I forgot that you need to go into Options in the drop down menu and check off a box that says something to the affect of search override for 2da files. Then close KOTOR Tool and reopen it so the new option is loaded.
Open up the utc file using the previous steps and then go into the Class tab. You should see a dropdown box where there is a class name, I think Carth's is "Soldier". When you click that box you should see your new class, so select it. Save the utc file in the override AS THE DEFAULT NAME IT GIVES YOU!!!
Now you're ready for testing. Load up your game, look at Carth's class and it should now be your new class. Please note that some changes require you to do a new game, I'm not sure if this is one that makes you do so or not. So if you don't see any changes be sure to try a new game.
Also this may not be the direction you're looking for but this should give you a good start, let me know if you need any additional help and I'll be happy to get you redirected.