There are a number of file formats associated with The Witcher (PC):
Starting with the .BIF / .KEY file formats you can extract the individual data files from any source in the game. With the .GFF file format you can then read and modify most of those data files, repackaging them when you are done. The creature, conversation, door, encounter, item, merchant (store), place-able, trigger, sound and waypoint documents provide additional details about those specific object types and how to interact with the corresponding .2DA files.
An article dealing with the locations of these files can be found on the Game layout page.