I don't really know why anybody would want to use ddraw rather than d3d - its almost certainly a waste of time to even consider using it unless you need compatability with an old system. This is true even if you want to make a 2d only game.
However its pretty easy to write a new renderer and plug it in to CEGUI, so feel free to go ahead! Its already got dx8, dx9, opengl, ogre, irrlicht - all you need is to create a ddraw render & ddraw texture class, and implement their virtual methods.
Your best bet is to use completely separate classes, as the implementation will be totally unique for ddraw, and you don't want to muddy up either dx renderer with legacy functionality (or rather, I use dx9, and I don't want it muddied up).
