A lot of PSU's only have one fan (I'd say that most do, actually), so that's not really a problem, unless the fan is woefully inadequate. Secondarily, I don't really understand how mounting another fan to it on the outside and not on on the inside (which isn't really possible anyway) would even help anything. It certainly wouldn't help cool the PSU any more.
It's possible? when I look inside my psu it looks kind of crammed I'm not sure how I could shove it in there
well ok then nevermind, I should just replace my other case fans
is 35-45 cfm good for a 80mm fan?