I am very familiar with this problem. You have a swayback.
A swayback happens when your front torso is longer than your back torso. Or, to put it another way, your center back length is too long.
Adding a skirt or other anchor will NOT fix it. You will still have bunchy fabric back there. At worst, it'll pull it straight and it'll look like your butt slope begins at the middle of your back.
The quickest fix is to run a horizontal dart along the back waist, so that you're taking in the greatest amount in the center and it tapers to nothing at the side seams. This way, it doesn't affect the length of the side seam in the back and you don't have to do anything to the front.
There's another, slightly more complicated way to approach it if you're comfortable with flat pattern manipulation. Here's an article that delves into the entire problem: