You may use the provided extractor to unpack the game archive to develop game mods. To repack the archive again use any standard ZIP utility after you've finished your mod.
Usage
scs_extractor archive.scs [output_path]
You may also click and drop the .SCS file onto the SCS Extractor to extract the files. For example drag the base.scs file onto the SCS Extractor. This will then extract all the containing files into the current location. You should do this in a separate folder to keep files tidy, just copy the Base.scs file (for example) into a separate folder along with the scs extractor and then follow the above instruction.
Download
Download the extractor here.