The KSDK comes bundled with the CMSIS core functions. You can use the 'NVIC_SetPriority(IRQn_Type IRQn, uint32_t priority)' function to set the priority of each interrupt. For example:
NVIC_SetPriority(PIT0_IRQn, 1);
NVIC_SetPriority(I2C0_IRQn, 0);
Please let me know if you have any trouble implementing the interrupt priorities this way.
Thank you,
Martyn