I have tried this in the past although not as good as pure liquid soap which is made from potassium hydroxide, since solid soap is made from sodium hydroxide, I think it works better solid.
I grated soap and added water and heat in a stainless steel pot until it melts, make sure it is quite diluted i think it's like ratio of 3:1 (not sure exactly). It thickens as it cools.
It works better with black soap because it burnt Ash used is predominatly potassium hydroxide, the high content of solid butter in black soap is actually while it is solid, hence the reason it melts very fast. By enlarge black soap will be a liquid soap if not for the high content of solid butter (in most cases shea butter).
HTH