eroge cg extraction and repacking

I was interested into extracting some images from a u-me soft game and translating the menu images into English before repacking them into the data file.
Thing is, extracting images isn't a problem, plenty of software for that, it's repacking that's an issue.
Anyone with any idea on how to repack data files into it's original format or do you need an individual programmer to write original software for a repacker for specific engines/encoding formats?