Who wrote the passage on the Poneglyph at Skypiea?
It seems that we finally know. Oda revealed in chapter 818 that...


  ...it was most likely Lord Kouzuki Oden that wrote down that passage. The Poneglyph were created by the Kouzuki clan and it was confirmed that Lord Oden was able to read the language and write on the stones.

 

 Later it was revealed that he himself had travelled with the former Pirate King, Roger. So chances are that he travelled along with Roger to Skypiea and wrote that additional paragraph.