Your not trying to extend the background. What you want to do is replace part of the existing background.
Yes, absolutely! I would like to replace the top portion of the existing background with the white sheet of the portrait.
There are many ways here are two quick ones. Made your image a normal layer
Added layer under it and fill it with a sampled color of you white background and converted to a background layer it does nor need to be a background layer though.
I add an empty top layer.
I add a white layer mask to your image's layer and painted black on it in the top let and center to hide some of the images background.
Switched to the clone stamp tool targeted top layer set sample current and blow in option bar sampler top white background area and pained in the top right corner
I can paint better then this example....Next time just your wife and baby please....