It is possible. Instead of reading data from WinCC, I imagine you would be communicating directly with the PLC. This will allow you to both read data and write values back.
Your PLC programme will probably have to be updated to allow for control from Ignition. We often create different control modes so the system has to be in ‘SCADA’ mode before it will action controls coming from the SCADA. If you are initiating control from SCADA, I would normally set a PLC register to a value and then make the PLC responsible for responding to the control and resetting the register. This means you don’t have to worry about problems with communications meaning a control value is not reset.