In mine Neverwinter Nights directory i see a zipfile with the title '2dasource'. Does anyone knows what it exactly is? Are they usable files for the game or somehing?? Can anyone explain that?

They are tables that are used to determine certain aspects of the game. For example, the classes.2da file tells the game the available classes to PCs and NPCs in the game, how many skill points they receive, which 2da files define what are class skills, etc.

Something you can do with it (in a cheating way for the game) is pick a basic class like a Fighter, modify it so it receives more than 2 Skill Points + INT modifer per level (like 8+INT modifer), increase the HP you gain every level from 10 to 20,
and make it a spell casting class.

After you change the appropriate files, just have the modified files in your Override directory. They should override any class changes from downloaded modules if they are in the Override directory. There's a tool that you can download that opens the individual files up in an easily readable format, or you can use Notepad++, or even Excel.