If you do not have any special merit or special omissions, write on the same sheet of paper, what other duties, in addition to basic, you could perform. Be honest with yourself. Do not start a conversation with a future employer with a list of obligations that you are due to lack of skills or time and then not be able to perform.
Choose the right time to talk to the manager. Do not start it, if you're not in the office alone or during an emergency work at the end of the year or quarter. Sign up to see him (it is better for an afternoon hour) or arrange with him separately about the time of the visit (depending on the level of the authorities and corporate policy).
Before the conversation, prepare a list of successfully completed projects and / or a list of possible duties. Managers are always busy, so it is easier if all stated in writing on a sheet of paper, which can be a couple of minutes to run through the eyes. Talk to the chief in a calm tone, and try not to let his disappointment in case of failure. Then, you may be able to return to this subject in a few months.
Do not expect an immediate response. Firstly, the head of the need to talk about it with their superiors. Secondly, the company is currently just might not be the right tools and right to refuse for such a reason will not every employer. Especially if you are really valuable employee.