Looking at the electrical characteristics of the hardware manual, the output high-level voltage for output ports P56 and P57 is min Vcc-2.5V. As in the case of other I/O ports, is it possible to switch these ports switch to the output levels of Vcc-1.0V (IOH=1.5mA) and Vcc-0.5V (IOH=0.1mA)?
No, it is impossible. The electric characteristics of these ports differ from those of other normal ports because of the difference in buffer's hard configuration. By setting a pull-up resistor, you can raise the output level to around Vcc. For detailed information, please refer to the hardware manual.