How to obtain and modify an AJAX request?

Question

I’m creating a supplement to my own study for the Chrome, in the add-on I modify several parts of a website. But now I need to modify a request ajax, the site makes the request before the extension script goes into action. I’ve done something similar but in localhost, I used to XMLHttpRequest to capture all requests ajax and modify them into a function.

But in the Chrome, I don’t know how to do extension capture the request "/event/original/" and do the return of "meusite.com/meuscript.php? ul=/event/original/"

How I capture a requisition ajax and the modified return to the remote site?