Hey Christoph... Usually fruits are best to add after primary fermentation. depending on how you want the beer to taste. The fermentation will scrub off most delicate flavors from the fruit.
The problem with using certain fruits is getting it back out of the fermenter, this is where others can help you with theri experience. Id add the berries to a flat bottomed tank just after fermentation and blend the green beer in with the fruit. At this piont allow the beer and fruit to sit for a week or 2 at temp, then cool the beer down and allow the fruit to settle down to the bottom. If you can add a stand pipe to your flat bottom tank that would help you a lot later when you want it out.
Also the fruits can cause the beer to sour, sometimes thats a good thing, if you dont want this you can use the mash tun and try to pasteurize the fruit prior to adding it to the tank. I think it may be easier to deal with if the fruit was divided up into small plastic bags while soaking in the hot water. Pasteurizing will also help to burst the cells of the fruit so it leeches into the beer faster.
Then you may have to use a screen in tube filter to get the beer out of the tank, you will still have a bit of a mess to clean up once your finished.