It's quite a laborious process, it's basically just copying and pasting all the right icons and text from the roman folder to the egypt one and renaming them so they get referenced by the game. I don't think you'll be able to get rid of the culture penalty though, but you can code offsetting bonuses into the government building as Dol Guldur describes in his two tutes:
https://forums.totalwar.org/vb/showthread.php?t=50433
https://forums.totalwar.org/vb/showthread.php?t=50439
.