Is it possible to set the voltage of multiple PWR-01 series units simultaneously via the RS232C interface?

Yes, with the VMCB (multi-channel) function, you can make the voltage setting for multiple PWR-01 series units at once using the GLOBal command.
However, there is a time offset in the response time for each unit. This time offset depends on the number of units and network conditions.
For the details, read the communication interface manual – chapter of GLOBal Command and Multichannel.