Using xkb to map a key combination to another

How can I use xkb to map a key combination like Super + Shift + C to another combination like Ctrl + Shift + C?
The question is about doing so with xkb. I know about autokey or xmodmap but I want to do it with xkb as other options aren't as stable as xkb in current versions of X.

Update

It'd be awesome if these mappings can be application specific. It'd be super awesome if these mappings could be system wide except some applications.