I've not seen a dedicated pump like this, but have read custom mods that have done this. Usual reason is the grey water tank fills way before black in most cases. Some have installed a pump to use grey to flush the toilet, thus 'transferring' grey to black. Others have discussed opening grey tank dump valve and black dump valve WITH END CAP ON to use the common manifold to allow grey to flow into black tank. Problem, some waste still remains in manifold when valves closed, so attaching hose will be a mess!
Simplest solution, wash dishes in a dishpan and empty down toilet.