Can a transformer be used to transform direct voltage and direct current?

For all intents and purposes, none. Transformers pass alternating current. Now, if you want to split hairs, when a direct current is initially connected to a transformer, magnetic field starts to build in the primary windings, and as this field builds, the lines of force cut through the secondary windings, MOMENTARILY producing an output voltage in the secondary windings. However, once the magnetic field is stable (within a millisecond or so) the output of the secondary windings fall back to zero. When you remove the direct current from the primary winding, the same thing happens again. As the magnetic field collapses, the magnetic lines of flux cut through the secondary, momentarily producing an output voltage. After the magnetic field collapes completely, the secondary output is zero. That is basically what you are doing with alternating current...inputting a positive voltage, then going to zero, then negative, then back to zero, building and collapsing magnetic fields so that it induces current to flow in the secondary windings.