I made one a while back for Mixxx v1.11.0. I'm currently not working on it until the next major release of Mixxx is finished. It's not perfect, but you might find it useful, and I have most of the basic functionality done. I know a couple of other people have taken what I've done and worked on it / improved it (including using it as a base for some DDJ-SB mappings, since the devices are quite similar), but you'll have to search the forums if you want those.
You can find it here (includes a description of whats done / not done): https://github.com/hrudham/Mixxx-Pioneer-DDJ-SR