The seamless transition is definetly possible however not always advised.
I'd always want to sit right down into the armbar after any hip throw.
My Judo instructor, (and a brown in bjj) stressed that getting control first was MUCH better than just going all out for the armbar. Even if its just dropping a knee on them to pin them a little bit (they are on their side, arm up in the air, and you are placing knee almost in armpit) not necessarily kne on belly.
Getting the control and position was far more important than just jumping into an armbar in his view.
It works, but chances are better people will escape it, because they know that arm is exposed.