Personally, I think Joe Quesada is a very professional man, unlike his DC counterpart, and would not integrate Superman into the Marvel Universe out of respect to DC Comics and it's fans, even if the Siegels and Shuster Estate went up to him and offered it to him. That and various legal reasons concerning DC's trademarks that I don't think Quesada would even want to touch upon.
Also, Superman in the Marvel Universe would just be awful. He really doesn't fit in there IMO. Marvel is a universe where asides from Spider-Man due to magic, every major superhero there has a public identity. Everyone knows who Captain America is, who Iron Man is, who Thor is, who Daredevil is, etc. Superman's secret identity, especially with the way he hides it, would look just silly there.
Metropolis would feel odd on account of every major hero asides from the X-Men, being based in New York City.
And a Marvel Superman would be incredibly gimped due to DC owning all of the trademarks concerning Superman. He wouldn't have the famous \S/ symbol, he can't fly, etc. He'd just be an incredibly strong man which Marvel already has with Luke Cage. And the only interesting villain of Superman's rogues gallery that would be transfered would be Lex Luthor, all of the other good ones like Brainiac, General Zod, and Bizarro are owned fully by DC.