Building an AA battery replacement
What you want to do is create a "dummy battery" that will fit inside the camera in place of the AA batteries and make contact with the battery contacts in that compartment. Here's an example of someone doing something similar: http://www.lumika.org/gear_a2e_batts.htm.

Building the dummy battery for your camera is really just a mechanical issue. The electronics design part could come in if you were trying to supply power with a higher voltage and needed to regulate it down to match the voltage output from the AA batteries (most likely this is 3 V for the two AA batteries in series). This will depend on your camera and the external power source you decide to use.