How much weight would i lose if i only ate fruit for a week
Whatever that number is, you'll gain it all right back. Crash diets NEVER work.
By eating fruits only for a week, you would probably lose 1 or 2 kgs. This is because, fruits have a low calorie but with lots of nutrients. Therefore, the diet is not advisable as you could end up with nutrients deficiencies and you are likely to add weights when you go back to normal eating. Check out http://www.livestrong.com/article/385633-how-much-weight-could-you-lose-eating-only-fruit/ for details.
Fruit turns into sugar in the body. Zero.
I don't think you can do it.