Is there an opening where the fan can pull in air from the outside? If there isn't, then, no, you can't exactly just stick a fan there, it would be pulling from a solid surface, which would probably make a lot of annoying noise, and just spread hot air everywhere. You always want fans in the front pulling air in, fans in the back pushing air out, and the fans on the side of your case can pull in or out (I prefer in) depending on the way your case is laid out and what gives you the best results.