Is it possable to create a new difficulty level after Master? if yes - how?

I'm not 100% positive, but I think that's hardcoded, which would mean no.

btw - where do you even find the written Master difficulty or any other one in the Creation kit?

Gamesettings.

It is hard-coded, but you could easily make a scripted version that adds custom difficulty levels. You can adjust the player in and out damage with perks which is all that the real difficulty level changes, so you could just add/remove different perks for each difficulty. Then just make the real settings all 1.0, so only the scripted version applies. You would just have to make a scrioted selection menu for it as well, instead of the difficulty level being selected in the normal menu.

Where can I see the Game settings window? (or how to get to it)

You don't really need to get to game settings if I am understanding him correctly. If you wanted to make a "new difficulty setting" all you would need do is make a book or something that adds a perk that has one entry point saying "Modify Incoming Damage" - "Multiply Value - 2.0", and that one would double the damage you take. Then make another entry point with "Mod Attack Damage" and Multiply Value 0.50.
Now if you read this book, the game becomes much harder. Combine it with Master Difficulty and you get a very challenging game.
You may also want a Mod Spell Magnitude by 0.50 for destruction spells.

Click on Gameplay (along the top of the main Creation Kit window), settings is the first option below.

This brings up the GameSetting box, in the filter window type difficulty*value. This will show 3 difficulty options, Default, Max and Min.

DISCLAIMER HAVE NOT TRIED THIS
There is possibly another option which might work. If the Default value was changed to 2, Min Value to 0 and Max value to 4. You would still have the 5 difficulty settings, but would the following occur on these new settings:

Novice now difficult as Adept
Apprentice now difficult as Expert
Adept now difficult as Master
Expert now 3 times as difficult instead of 1.5
Master now 4 times as difficult instead of 2

I find modifying the difficulty with simply changing the damage values lazy and just... bad. I know this is how the vanilla game works as well and I always play with the master difficulty. It works in the early game (even though some fights are just impossible without exploits), but you can really see how broken it is later. It takes forever to kill tougher enemies while they hit very hard. This sounds like it's not a problem, but it gets ridiculous at some point. Not to mention the followers aren't nearly as weak as the player, and most of the traps and shouts are useless against enemies. Try the fire breath on a high level dragon and tell me you see a change in its health bar. The way the difficulty is implemented is just bad.

This being said, it's really hard to change the difficulty efficiently, especially if it's that limited. What I would like to see, is improved combat, that would actually require some skill instead of hack&slash on easy difficulties and exploits on harder difficulties. Clever use of shouts, skills, poisons, potions, traps and environment wouldn't hurt either. A good example of adding difficulty in a positive way are the realism mods (haven't tried them though).

All of this is just my opinion of course, but I'd love to see a well-made difficulty mod.