Using the Arduino compiler, is it possible to use Atmega2560 interrupt pins that are unused by Mega?
If you are not afraid of getting your hands dirty by touching hardware
registers, you can access those pins bare-metal style, using port
manipulation. You will have to read the datasheet,
specifically the section “External Interrupts” if that's what you are
after.

This works with the stock Arduino core (or even without core), which may
be interesting from a maintenance point of view, but it's not as easy to
use as the Arduino interrupt API.