"fits in seamlessly" is correct and usable in written English.
You can use it to describe how something seamlessly "fits in" with the rest of something, meaning it seems like it belongs and complements the other pieces. For example, "The new design feature fits in seamlessly with the old architecture."
Pretty good and it fits in seamlessly.
"He's one of those guys that fits in seamlessly," Chicago Coach Joel Quenneville said.
Its spell is all-consuming and decadent, with a glib dash of funk ("Constant Conversations") that somehow fits in seamlessly.
This baritone projects an index of styles and suavities ranging from Joe Turner to Joe Williams; as vocalists go, he is musicianly and fits in seamlessly with the band behind him.
It all fits in seamlessly with Fire TV's other content and its user interface.
Just like Google introduced sponsored search as an advertising model which fits in seamlessly with search results, it doesn't take a whole lot of imagination to see how Facebook could build brand and discovery-based advertising into its product which will be useful to members.
I am getting a bar of Irish Spring and making secretly ready to take an extended sojourn to the Emerald Isle, if necessary, where I will fit in seamlessly.
But that, obviously, was the point: The goal was to make most of what is new disappear, or fit in seamlessly.
There are also dueling Chinese video gamers, a grocery manager trying to catch shoplifters so he can afford a pool a person can swim in without actually going anywhere, the lost crown of Ethelred the Unready, various doctors, a psychic nurse, murder victims, a hamster and more, all of whom fit in seamlessly.
He has fit in seamlessly and contributed in more ways than Manager Joe Torre could have known.
