The system consists of a closed loop from the output of the pressure transducer to the PLC input, through an internal resistor in the PLC to the input ground, and back to the ground on the transducer.
The pressure transducer creates a variable current in this loop of between 4mA and 20mA depending on pressure. Usually more pressure equals more current.
There is a converter in the PLC that measures the voltage drop across its internal resistor, and converts it to a pressure reading.
What you have to do is trick the PLC by creating a current source to replace the transducer.
This may work, but could have issues. Try at your own risk.
The quickest "dirty" way to do this would be to connect a large value resistor between the PLC power supply and the PLC input.
The input's ground terminal will need to be connected to the PLC ground to complete the loop.
Connect an ammeter in series with the resistor to view the current.
Lower the value of the resistor to increase the current to get the reading you require.
As advised earlier, only do this if bypassing the pressure transducer will not create a possible dangerous situation for the machine, operators, or other people, etc.