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.
That's typical HP.
There appears not to be any more opening for fans other than the one in the back. However, Drop the front off by taking the front cover mounting screws out or popping the retaining tabs and dropping it off. There on the front frame, if it's made for it, should be a spot for mounting a fan. Going on if it has one, the fan goes on the inside of the case drawing air in. You would have to drop the front off to get the mounting screws in anyway.
If you have a repair shop close by you might want to venture over and see if they have an empty case they may part with for free or cheap. Take 20 dollars with you. I've walked out with a ful height server tower for 20...