Theoretically, it is possible. However, I would absolutely not advise it, because liquid metal is meant for high-end, and the boxed cooler is not a really good cooler. It would be like fitting a VW with a V12. It would be a complete waste and totally unnecessary.
Yes, you can use liquid metal. But keep in mind that you have to remove the already applied thermal paste from the CPU cooler first (I use cleaning solvent for this myself).