I think there's no such a program provided by Canon or someone else
BTW, sRAW is quite different from other RAW formats. It's not RGB data from sensor, but already demosaicked raw file, resized to smaller size and converted to sort of Y Cr Cb format
BTW2, you won't get better compression with dng, because both cr2 and dng are using lossless jpeg compression. However, RAW+JPG is actually (RAW+JPG)+JPG, because you have raw image and jpeg image in a cr2 file + jpg image in separate file (at least in older Canon cameras, like my 400D). Compressing to dng, you get rid of jpg that was embedded in cr2, so dng IS smaller than cr2, because it contains only a raw image (+ some metadata and small jpg thumbnail)