Hi,
pin interrupt and function is selected using PORT.PCRn register. The pin direction is then set by GPIO PDDR register. See more in chapters 12 Port Control and Interrupts (PORT) and 13
General-Purpose Input/Output (GPIO) of teh device Reference Manual.
Anyway I2C pins should be bidirectional with open-drain output, which is not possible to set on normal GPIO pins of this MCU.
BR, Petr