Voltage-controlled conductance and switching induced by single molecules or atoms are ideally studied in scanning tunneling microscope (STM) tunnel junctions. While the objects under consideration are mostly used in their original form, little is known of the possibilities of in situ adjustments of their properties. Here, we evidence properties of a tunnel junction made of a Ce atom/cluster built by atomic manipulation on Au(111) at a temperature of 4.6 K in the presence of H2. The conductance through the object is characterized by a switching voltage corresponding to an opening or closing of an inelastic electron tunneling conductance channel at 50 mV for a Ce atom and 140 mV for a Ce cluster and by charging. We demonstrate that the electronic properties of an STM junction can be engineered in a simple way by in situ guiding of the H2 pinning at an atomic cluster.