A conveyor belt is moving, so it has kinetic energy. When a stationary object is put on the belt, it starts moving and also then gets kinetic energy. But, the object before going on the conveyor belt didn't have potential energy, so where did the kinetic energy come from? The answer is that the engine driving the conveyor belt is converting chemical energy stored in gasoline into the kinetic energy to move the belt and the things on the belt. (The way an engine transfers energy is the subject of college courses, but in short, it takes chemical energy and converts it to potential energy, which then is converted to kinetic energy.)