Can I add an exhaust fan to bathroom without exterior wall or roof access
Without seeing the house I cant tell you. It sounds like you are talking about creating a soffit which is going to be alot of labor.  

You have some other options.  If this is just a 1/2 bath you can but a vent-free vent that uses a charcoal filter for smells.  If there is a bath then you can get a small dehumidifier.