In the ‘777 Captain’MANUAL Part II – Aircraft and Systems (CS777CAPTAINII_MANUAL_PART2.pdf, page 111/112 (starts on the bottom of page 111, but all the important info is on page 112)), it explains how to add you own charts.

I'm not sure if you can use the navigraph ones in there, but I assume you could (as long as the charts are bmp, jpg, jpeg, gif, png, tif, ico formats).