Nearly any game can be modified if you know how. Actually, once you know how to edit the game's archive files, such as Blizzard's MPQ files, you can actually edit the files or even extract them for play elsewhere. Truth is, you can do this with most games. The issue lies in companies that use proprietary or modified codecs. For example, Blizzard's video files (modified MPEG codec) won't play right if you just pop them out of the MPQ, they have to be converted or the appropriate codec needs to be installed (easily found online). Hasbro Interactive did a similar thing with their Star Wars Monopoly title years ago. The video files (scenes directly from the movies) were encoded upside down and backwards (in other words, flipped both vertically and horizontally). While they would play normally in game, they would appear backwards and upside down if you tried to open the files directly.
Some games actually give you most of the files and encourage modding and even tell you how to do it (examples being Neverwinter Nights and Dragon Age, both Bioware games).
As for getting into certain archives, some are simply renamed ZIP or RAR files, and some use a proprietary archive format which require special viewers to view and edit.
