There's no reason you couldn't use a wheat beer kit and add fruit flavoring and make a very nice beer, but whether it will taste like Leinie's Berry Weiss will depend on the fruit flavoring you use, and the kit's ingredients - not all wheat kits are the same, and not all wheat beers use the same recipe.
In my experience, you want to add fruit flavoring JUST before bottling. That's how I did my
Black Acre Berry and it worked out perfectly.
If you're using real fruit and not extract, some people add the real fruit during the first boil, after the boil, during secondary fermentation, or just prior to bottling. There is fierce debate over whether to freeze the fruit prior to using it, or boil it, or both, or neither. Some people sanitize the fruit with starsan (not a recommendation of mine, but different strokes for different folks.)
I'd suggest that if you want to duplicate Berry Weiss, CONSIDER adding 4oz lactose to sweeten your beer, and knocking off some of the hops in the kit in order to decrease the bitterness.