It is possible, and has been done before. But the people who did, weren't that happy with the performance afterward.
The pump on CLC water-coolers is quite weak, barely capable of powering what it comes with at stock. Adding in a GPU block and radiator would be too much restriction.
Also, CLC coolers often use Aluminium rads, which is a metal that is known to cause Galvanic Corrosion when the typical water-cooling metals (Nickel, Copper, Brass) are present. You could end up corroding your GPU block unless you take precautions.
My advice, just go full out with a custom loop. Get something like a Raystorm D5 kit, then add in your rad and GPU block.