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 like 7-Zip after you've finished your mod.
scs_extractor archive.scs [output_path]
You may also drag and drop the .scs file onto the SCS Extractor to extract the files. Dragging the base.scs file onto the SCS Extractor will extract all the containing files into the current location of SCS Extractor. So to keep extracted content out of game's original directory, you should create a separate folder, put SCS Extractor in there and then drag over the .scs file you want to extract.
Download the extractor here.