Modifying a squashfs,

Obviously, I don't want to actually modify a squashfs. What I would like to do though is take an existing squashfs, a set of files and create a new squashfs which is identical to the old one except that the files in the set either replace similar files in the squasfs or are just added if there is no similar files. OK, that last part sounded weird. So let me give an example: There is a squashfs called mfs.squash. Inside it there is a file ./a/foo. I want to create a new squashfs which is identical to the old squashfs – except that there is a new file a/b and I overwrite the ./a/foo with one of my specification.