You are raising a question based on a hypothetical situation. Before clarifying your doubt, I want to mention few things here. From the brief, it is clear that you want to gain money for your expenditure and don't want to depend on your parents or someone. It is fine. But joining a lower post which is not interested and preferring to get rid of it as and when you get a better position of your choice. Not a bad idea but may not work well unless you put your efforts sincerely in addition to attending the present job. Once you join a job, you won't be left easily without work and you have to take the load and may not be in a position to focus on it. If there is no need for you to join a small post, better you work hard to get a good job by attempting the relevant examination or interview.
Coming to the doubt raised by you here, when you join any Government Department , you have to apply for other examinations/interviews through proper channel only, in strict sense. Otherwise, an intimation top the department prior to applying the examination is to be made to avoid delay in transmission of the application through the departmental channels. This helps you to apply for 'No Objection certificate' from the present department while leaving it on resignation, in case of selection to a new post. In some cases, it would be treated as 'Technical resignation' and the service rendered in the previous department would count for pension and other benefits in the new department. Generally, one month notice is to be given to the department if you wish to resign and join a new post. This is applicable for all cases including joining a job in Banking sector