The NMI interrupt support was removed in Sept 2015. First in the main Arduino core and then in our fork. I can't remember why this was done exactly.
You can see the repository commit here: https://github.com/SodaqMoja/SodaqCore-samd/commit/6538f8950bd787e6c8d78df966d295cff1dd49dd
It seems all that you need to do is modifiy the
EIC-NMICTRL.reg to specify which detection mode. You probably want to attach another interrupt first or look how Arduino configures the interrupt clocks.
Then you need to write the NMI interrupt handler (this can exist anywhere in the code) which at a minimum should clear the interrupt flag:
void NMI_Handler( void )
// Do stuff
// Clear the interrupt
EIC->NMIFLAG.reg = EIC_NMIFLAG_NMI ;