Is it possible to apply +/- voltage to PLZ-5WH2 series by a positive and negative output power supply? (i.e., Can PLZ-5WH2 receive +400 V at the positive DC INPUT terminal and -400 V at the negative DC INPUT terminal?)

Yes, the maximum operating voltage of PLZ-5WH2 series is 1000 Vdc and the DC INPUT terminals of PLZ series are floating.
Therefore, you can apply +400 V at the positive DC INPUT terminal and -400 V at the negative DC INPUT terminal (800 V between the positive and negative DC INPUT terminals).