You can change loot quite easily by modifying your treasure.xlsm
Edit: Found it in the unpacked files. Public/Main/Stats/TreasureTable.xlsm
I guess editing frequency would make the item drop more and the rarity decides of what quality it should be. However, how do you make the file affect your game?
Edit2: Found the answer here:
http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=507354&#Post507354"Almost; you should drop them into Data\Public\%yourMod%\Stats, not Data\Mods\%yourMod%\Stats".