Can a newcomer to an area or a new employee be referred to as a gaijin? They are an outsider initially, when they arrive to a new area or job.
no, the "outsider" terminology used in Japan is entirely about nationality, never about "outside the company". People newly hired to a job or newly part of some group are called 新人, literally "new guy" or "newbie". Another common word used to describe such people is the adjective, 青い, which is used because it is meant to represent that the person is "unripe" or inexperienced. It can be meant pejoratively, or simply protectively, depending on context.