In Resident aEvil Apocalypse Jill Valentine, Carlos Olivera and LJ joined Umbrella, then in Extinction Carlos and LJ were part of Claire Redfield's convoy, why and how did this happen?

They didnt join Umbrella. They disguised themselves as Umbrella employees to get Alice out of the Detroit facility. :) How, why and when LJ and Carlos joined Claire's convoy, I dont know though.