As Stumpkiller said, you need a different string the correct length, not a string that will stretch more. Look at the string to see how many twists are in it. You can go down to about one twist per inch. If that won't get the length you want, will need to get a longer string. I would think the Robertson's would exchange it for you.
With low stretch strings, it's much more important for the string maker to hit the correct length because the skinnier high performance strings don't move much per twist, i.e., they require a lot of twists for a small gain or loss in string length. Good luck.