There is no pin mapping for PJ2~PJ7.
Actually there are 16 pins not mapped for the Arduino Mega board. Not only are they not mapped, there is no wiring traces from the processor chip to any connector. The Seeeduino mega board does wire these 16 'extra' pins to an additional connector, but because the Arduino core software does not map these pins one has to use direct port access commands to mode/read/write to these pins. These 'unreachable' pins on the Arduino Mega board are:
Direct port commands are explaned here: