Why would I need to?
Stopover only in Europe?
How long can it be?
You might need to if your plans change. US is very rigid.
Stopovers are of any length of time.
W/ US, you can officially stopover at any OneWorld hub, but practically, you can likely convince an agent to do it anywhere.
In addition, if you spend enough time on the phone, you can likely finagle a long fun routing. Their CS isn't the smartest...