Should i trade willis McGahee and felix Jones for Addai?

Absolutely. You get the value of two solid backs and it only takes one slot on your roster. Felix is great but you can't play him confidently from week to week because you never know how much time he's going to see. McGahee might still come on, but I think that even at his best he won't come anywhere near the potential of Addai. With Westbrook, Addai, and Johnson in your flex you can take most leagues (depending on your receivers and Quarterback of course :).