Even though the role may not be exciting, accepting this offer would give you an opportunity to have a foot in the door. By that, I mean an opportunity to network within the company, understand its politics and structure intimately. This will also give you access to opportunities that are invisible to those on the outside.
It is easier for a company to transfer an individual from one department to another rather than recruiting externally. By working there, you are a known quantity.
If you excel at the job, even though it is not your dream job, you are positioning yourself ahead of the external candidates.
By refusing the offer, you may be giving the impression to the hiring manager that you are not interested in working for that company. This will stick with him or her and may affect you negatively regardless how polite you are on declining the offer.
The hiring manager has invested time and money in all the work it took before extending the offer.
Therefore, I would not only accept the offer and start working, rather than hope, on the possible internal transfer.