OBTHOMAS
If you still want to add a fan it can be done. The Norcolds use 20VAC/60Hz produced with a built in inverter , a multitap transformer and relay to switch between the 12VDC and 120VAC supplies., ensuring neither is on at the same time and to use the appropriate transformer taps.
If you search back on my name a [few] years, I posted a schematic I made to take the motor voltage [20VAC] and produce 12.8VDC of about 500mA to drive a 12VDC muffin/computer fan. It worked well for many years [15-20] untill we finally decided to change the fridge, partly for the Danfoss unit and also to get a larger unit.
Other wise i suspect we would still have that old Norcold.
One thing you might try is purchase a muffin type fan ,4", that will operate on 24VAC. It will run slower than if it got 24VAC but should still do the job.
Find the motor power leads and solder a pair of wires to those leads to run the fan. Use a fuse just in case.
I will repeat here, use only one fan and will be dead quiet. You will barely hear it.
DO NOT use Two smaller ones as they will most likely make a lot of noise.
Taking the fan power from the motor leads means the fan will run ONLY when the motor runs. The little extra load is small enough the unit never noticed.
Upon reread I realize you wanted the fan for fridge interior air circulation. i used mine to remove the rejected heat from the cooling coil to outside of the cabinet. If you only want the interier circ. fan then a 2 or 3" would do the job.
If the cabinet does not now have a fan to remove the fridges reject heat then consider a 4" fan also to evacuate the cabinet. If the fridge cannot get rid of the heat because it is trapped in the cabinet then it cannot cool effectively, running longer and more often. I substantially improved the fridge's performance by evacuating the cabinet.