There's the offline mod tool
[You must be registered and logged in to see this link.]
There's Horizon, what you should use for file extraction and rehashing and resigning
1. Open your usb (with MW3 SS MP save on it) in horizon and extract the Split screen multi-player progress to your Desktop
2. Open the file with the offline editor and edit all the stuff to what you want it to be. Save when finished. You can also unlock all by clicking the "Unlock all attachments and camos" button
3. Drag the file on your desktop to Horizon and rehash and resign
Please rep if this worked!