Well, no/yes, when you copy the subclass you'll see all the things bootstrapped to it on your copy. One of those bootstraps will be Circle Spells (cDrdCirSpe). Knowing that you can now move over to the Class Special tab and make of copy of Circle Spells. You'll see where the original bootstraps at least 64 spells and puts a condition on each of the bootstraps that looks for an index (this being the index on the dropdown list). You can simplify your copy of Circle Spells and bootstrap only those you want without conditions and remove the dropdown array (easiest) or insert your new circle into the array and bootstrap your new spells and go through all the spells and make sure they use the right index on their condition (hard and hint: the array index starts at 0, not 1).
So at this point after saving and testing you'd replace cDrdCirSpe on your copy of the new druid subclass with the new Circle Spells you just created/copied and it should be ready to go.
So, a bit of juggling and a tough subclass to crack.
Creating a new Paladin Oath is a lot easier. You can copy any existing oath and see how it bootstraps oath spells I'm not aware of any of the Oaths that change out spells like the Circle of Land does for the druid so they are usually a flat subclass that just bootstrap the spells and the other specials.