I have a Python script for sale and you will save $100. The only caveat is that you would have to tweak the script for every DAT file, but I've added detailed comments and explanations throughout the file.
In essence, it takes 2 txt files (exported from L2DatEditor) chops them to pieces based on certain criteria (be it Item_id or Skill_id or whatever), and then takes values/text from 1 file and puts it in the other file. Also has certain fields for conditions that you can also use.