SPK File Format
Quest for Glory 5 stores all game resources in SPK files, which sit inside the game's data directory.
Extracting files from SPK archives
Currently this can be achieved using a plugin for the Game Extractor software package by watto, available here: http://www.watto.org/extract/info.html. The required plugin for reading Quest for Glory's SPK archives can be found here: Plugin_SPK_2.zip.
Currently this plugin only supports extracting files from SPK archives, however if you copy any modified files under the QFG5\Data\ directory following the same directory structure as the SPK archives, the game will use these instead of the original files.
You must install the software then, under the install directory open up GameExtractor.jar in WinRAR – http://www.rarlab.com/download.htm. You must add the extracted plugin file (Plugin_SPK_2.class) to this archive. After following these steps, you should be able to open the SPK archives from your game's install directory (QFG5/Data/) using Game Extractor.
An alternative solution for unpacking SPK files exists in the form of a Python script: unpackspk.py
This can be run on any platform with Python 2.7, using:
python unpackspk.py path/to/file.spk destination/path