No, not at all! In a competitive recruitment it is expected that you will have letters of rec for all of the chapters, not just some. And besides, believe it or not a lot of us have great (if not best) friends in other chapters. And while I cannot speak for every sorority woman, I love the idea of women going Greek in general, just as much if not more than I love the idea of them in my sorority. So no, in short I would not say that would be offensive at all. If it were me, I would love an excuse to go talk to some of my old friends. Just say that you're rushing at Competitive U, were securing recs to all of the chapters, and that you have looked high and low and you were wondering if Sally Sorority by chance knew of any KKGs that would maybe be willing to write a rec for you. Also don't be afraid to ask any men in your life if they knew any KKGs in college. If they were Greek (and even if they weren't) it's completely possible that they do. Best of luck and let us know how it goes.
ETA: Titchou and IrishLake beat me to it.