You want to measure temperature using the PIR detector?
Usually in a PIR sensor the pyroelectric detector is a pair of detectors internally, connected in opposition so they cancel. The output is provided through a FET buffer transistor also built in. The image of a moving hot or cold body causes a change in the balanced output that can be detected. The sensors used respond to change in temperature, rather than temperature, so a fixed scene will settle down after a few hundred milliseconds, and the system is relatively unaffected by ambient temperature or infra red. The charge on the detector device is varied by the incoming radiation, like varying the charge on a sensing capacitor. Note these sensors nominally work from UV to the THz radio range of wavelengths, so a filter defines them as long wave IR.
If there was only one internal detector this could be used as described below, but I doubt you could open the transistor style detector package and change the connection without destroying it. It may be possible to externally block off one sensor somehow from the field of view. I am only guessing about that.
In a non contact temperature sensor (also known as a radiometer) the detector has to be rearranged so that the sensor is alternately viewing a reference temperature and the sensed temperature. This is done using a mechanical mirror switching a few times a second in some designs. The signal will be then a square wave with amplitude according to the temperature difference, and this difference can be extracted using a phase sensitive detector to generate a DC voltage equivalent. This can then be processed to provide actual temperature, though the field of view and the emissivity of the object being measured could be an issue..
Note that the optical components like filters and lenses and mirrors have to work at far infrared wavelengths. If you use the fresnel lens out of a PIR sensor, it will have a wide angle field of view. As far as electronics in the PIR sensor is concerned, none of it is relevant to this task. Basically we are left with the fact that none of the components in a PIR sensor is of much use other than the filter when attempting to make an IR temperature sensor, though the detector itself is related. A pyroelectric detector without the opposing pair can be used, but they seem relatively expensive. IR thermometers are available very cheaply so it is unlikely to be worth making your own, except as an exercise.