The thing is that heating isn't needed for your purpose. I make all the time "liquid soap" by adding water to solid homemade soap.
Just stick in the blender (make sure you have some glasses on, you don't want to get your eyes splashed).
Blend and keep adding water until the consistence isn't gooey anymore and becomes watery (actually, at this time, it may be needed to add a little more water, since it continues to thicken over the next hour).
Normally, liquid soap has a different composition than solid bars and this recipe may not have same consistency as commercial liquid soap, but it's good enough for me.