As it was mentioned by KLouis, there are many cheap options to get from FRA to IST. I would check those before booking an award flight, as it could be a waste of miles.

To contribute to the thread topic, for anyone using search in future: DBV is a tiny airport, with pretty much no layover facilities. There's a small lounge with limited hours. If your layover is anything over 2 hours, I would give it a miss. Let me put it this way: I'm Croatian, and I would think twice before connecting to IST through DBV Actually, I'd only do it for the fun of it, since I'm a crazy FlyerTalker. But not for normal people