You sure can. There is a huge range of computer cooling fans available in all different sizes. The bigger the fan the more air it moves and the better the cooling.
Our Isotherm fridge came with a 12 volt fan which made more noise than the compressor. (!) I fitted a second 12v fan, but wired the 2 in series, so each runs at 6 volts. One fan "pushes" cool air onto the condenser, the other "pulls" hot air away from it. This setup proved to be more efficient than the original single fan and now the fridge is barely audible.