Edit the L2.INI file using a text editor to include a new line reading
"Cheat = 1". (Don't include the quotation marks!)
The position of the line in the file may be significant and then again
it may not. I placed it after the "SoundName = ..... " line.

You will find a new option is available to you on the preferences screen
once inside the game. With Cheat Mode activated you are able to move
between levels as if all have been completed.