yes, MEGA has 6 interrupt pins, but for pins corresponding with outputs it is not possible to access them from outputs screw terminals, because these are outputs !
If you want to use for example /D0/INT.0 you have to use pinheader X1 where the corresponding PE4 is, to access directly the processor pin. Please note, that the pinheader is 5V only so you have to use for example divider from your 24V. And you have to properly configure this pin as input in your FW application.
Please think twice before connecting the pinheader