I am in the process of changing careers.  There is a certain position I want at a company, but I cannot yet meet the qualification requirements, as I need to pass some professional exams first.  There is another position at this same company which I am qualified for.  I was thinking of applying for this second position just to get my foot in the door and learn the business in the hopes that I would be able to move up after passing the exams.  This second position is not entirely related to the first in terms of necessary skill sets.

Assuming I get an interview for this second position, my question is:  What do I say when the interviewer asks me why I want this position?  The only reason I' m interested is just to get my foot in the door.  I think I would enjoy this second position, but it' s not something I want to do long-term, and if I was not able to move up after passing the requisite exams, I would start job searching again.