Remotely triggering a PIR sensor, or it's controller, via IR signals is not a feasible option. PIR sensors are designed for human body heat ranges, and motion across zones, over time/states. See Can I electronically trigger a PIR motion sensor? for info on that.
Since your board is already designed, and assuming that you have no other inputs available, you are trying to turn the GPIO defined for the PIR sensor as an input for multiple things. This can be done, but it will require some hacking.
The NCS36000's OUT (or LED) output is a 10mA max push-pull output. I am not sure if you are using the xLED_EN pin, so let's assume you aren't. Also assuming it's directly connected.
Use a Diode OR pair. Since Out is active high, this works well.
simulate this circuit – Schematic created using CircuitLab
If you want a unique signal, if your input is a ADC capable pin, add a pull-up to the switch, so you create a voltage divider. A voltage lower than the PIR OUT's level would mean it's the button.
Or connect any active high circuit to the switch instead. A 555 Timer in one shot mode, or another microcontroller. As the PIR sensor only pulls OUT high based on motion, for a fix amount of time, a set of multiple quick pulses or even a single quick pulse would signal that it's the button. The PIR sensor takes priority. You will need a timer or polling to keep track.