answer:It might work, but you’d probably want to re-constitute it in some water or milk before mixing it in with the cream. I believe that creme fraiche made this way has to sit for a while to thicken up, one internet source says it has to sit for at least 8 hours. If you’re going to wait for 8 hours, you could find the time to go buy some buttermilk? The other way I’ve made Creme Fraiche is using a mix of sour cream and cream. I don’t remember the proportions we used, but just slowly mix in cream into the sour cream until it’s at a good consistency. It’s ready instantly, and it’s pretty darn yummy, although since I’ve never had the real thing, I don’t know how it matches up. What are you planning on eating it with?