"impossibly beautiful" is correct and usable in written English.
You could use it to describe something that is so beautiful that it seems impossible or improbable. For example: "The sunset was impossibly beautiful."
It's all impossibly beautiful.
They all sell an impossibly beautiful future.
Atop Poon Hill, we found an impossibly beautiful panorama.
The dunes are trackless and endless – and impossibly beautiful.
After all, Hannibal is an impossibly beautiful show.
Nabokov's sentence is an even-more-excellent-than-usual example of his impossibly beautiful descriptive imagination.
In its light stand two impossibly beautiful Nigerian prostitutes, nearly naked in bras and thong underwear.
But during one late-evening commute home he notes an impossibly beautiful young woman hanging, fast asleep, from the strap.
He was alert and impossibly beautiful, with a single dimple and eyes open wide as satellite dishes, receiving everything.
She is the impossibly beautiful, naturally aristocratic, charmingly accessible babe next door -- a celebrity friend.
It is impossibly beautiful - and the rum cocktails consumed en route help a little too.
