Can I change my 2011 iMac's graphic card from AMD's to nvidia Quadro?

I don't even think the hardware would be compatible. Quite frankly, while you can open up the system and tinker with it, you have to worry about two things:

1. Form factor - The card is probably a different shape. Apple partners with other companies when designing a system and the fact that they put an ATI in there says something about the engineering of the machine.
2. Power & bus interface - These are likely less of an issue, but something to be aware of.

The iMac is a turnkey system, not an upgradable machine, like the Mac Pro (or, gasp, a PC). I'm sorry that this is not the answer you want to hear, but I don't think that Apple will do it. That said, it can't hurt to ask.