Can Chirp Waves be sent using JSN-SR04T?
As I googled the sensor, I found various sites stating, that a JSN-SR04T behaves exactly like the HC-SR04. The difference is mainly the water prooving.

So the answer is: No, you can't do this. The square wave, which drives the ultrasonic transducers, is generated by a hardware circuit and is not meant to be changed. To output a chirp wave you would need to rebuild the signal generation and measurement hardware (leaving only the transducers unchanged). At that point you may just buy the pure ultrasonic transducers and build the rest yourself.