Absolutely correct, no overclocking allowed except for Dell Alienware systems. Nothing can be done here.
Stock fans, it is misleading statement - there are only CPU fan and an exhaust fan, this is it. You can try to improvise and place small, like 80 mm fan to the front panel, maybe you can even fit 2 of them, this is it, unless you drill the holes on the side panel, or move your XPS into new case. Those are your all options.