When changing the sheet type of a standard journal to a Campaign Codex sheet for the first time, there is the option to bring the content from the standard journal to the Info tab of the Campaign Codex sheet.
A dialogue will pop up to confirm the choice, and then, if selected, all the text pages from the converted journal will populate the Info Tab of the Campaign Codex sheet.
There is an option to create a Campaign Codex sheet for each page within a journal using the API.