Let's crash Ceres into Venus.
Nope. The impact would vaporize all that water and send it back into space.
Put a giant shade between Venus and the sun, and drop comets rich in water and organics.
Getting rid of all that CO2 and sulfur might be a biatch, though. As well as the retrograde rotation. And no real magnetic field...
Oh hell. Let's just terraform Mars - it would be a MUCH simpler job than Venus.