20 (SDA), and 21 (SCL) pins for the I2C bus and interrupts

The 20 (SDA) and 21 (SCL) pins in my Arduino Mega is already being used as interrupts for an encoder. Now I want connect an Arduino Uno as a slave through I2C so is it possible to use the same pins?

Or else can I define another two pins as SDA and SCL?

Only SCL/SDA can be used for I2C, as there is hardware within the chip to support it.
Can you move the encoder pins elsewhere, to a couple of pins with PCINT support?