Page 5 of the Core Rulebook, in the sidebar, states that if the character does not qualify for a given career, he can go to the Draft or enter the Drifter career. Book 6: Scoundrel has the Scavenger and Wanderer careers which, similarly, one can enter with no need for qualifications.
Because they need no qualifications, it would seem, anyone can join and rejoin these careers at any stage during chargen. Personally, I rule that if the character is forced into the Drifter career, he has to make up a really entertaining back story. My favourite runs:- Characters is off on shore leave, meets a gorgeous woman, gets separated from his unit, goes to her place, has a glass of wine - and his unit buddies find him three systems away, six months later, living in a Bohemian colony of artistic types, with a beard and a new tattoo, and no memory of who he once was.