Is the top of the table shelf open to air, or is it closed off like a box?
If it's the second, putting a fan on it (let alone where you're gonna connect it to..) will not help the heat problem. The PS3 blows air out the back. That's how that cooler is designed when the cover of the PS3 is on the top. If you have it 6-7 cm from the wall and it has a roof on top of the console, it has no room to breathe. Putting a fan on top of the fan, or on top of the hot "top", will not help air flow. You will just continue circulating hot air onto the PS3, because the hot air blowing out the back has nowhere to go.
If you think it's running too hot, find somewhere where the top of the console is open, or it has more room in the back to dispel air.