Is it possible, when at war, to get a county that is not in the original claim?
For instance, you want to take Scotland but have only the claim on Galloway, you press it. When you won the war, you'll only be granted Galloway, no matter how many counties you have taken. So you fabricate another claim, but that just makes for huge "break truce" penalties going war after war until you have the whole thing. Is that a way to add a claim when already at war?
No. What the CB at the start says is exactly what you get, no more and no less.