One on probation is a “probie” and one who is new is a “newbie.” other diminutives such as “swabbie” (one who swabs a deck?), “freebie”, “cabbie” and the like.
But the b is present in the root-word for probie, swabbie, and cabbie: they’re formed simply by the addition of -ie, which is typical of English diminutives (and similar derivatives), to the original word or its stem. In freebie and newbie the b is intrusive, and its origin is somewhat mysterious.
OBTW, old discussions: