I have 4C hair and the way I stretch it is by braiding it overnight. It doesn't come out straight but its less tightly wound than the natural hair. Try it :) I also saw that African braiding helps > https://www.youtube.com/watch?v=u3WvYqdCRgs https://www.yout ube.com/watch?v=Uek1n8Bizzw
The way that I elongate my 3c hair is I tie my hair into a pineapple when I dry my hair in a microfiber towel.There are products like Marc Anthony Strictly Curls Stretch- I bought it but I haven't tried it yet
At night I'll either wash my hair or use a spray bottle full of water to soak my hair wet. Then I add a leave-in cream and an oil on top to seal in the moisture. I braid my hair into about 3 or 4 large sections, nothing special just braid it down to the end while it is soaking wet still. The next day, grab a wide tooth comb (or just use your fingers) and after you unbraid the sections, separate them and lightly scrunch your hair with a little bit of a cream or oil in your hands. Your hair will still be damp, and it should stay like that throughout the day without frizzing up or shrinking! hope this helps.
There are products like Aunt Jackie's Curls & Coils Don't Shrink Flaxseed Elongating Curling Gel that you can purchase at Curlmart.