I have plans this weekend for finally sitting down and making my first beer; a hefeweizen from extract. I plan on adding some frozen raspberries to it eventually, knowing this should usually be done in a secondary. Is there a reason why it should be done in a secondary, and the fruit can't just be added to the primary fermentator once the fermentation is done, then left for a while? Or even added at the beginning of the brewing process? Is it due to fermentable sugars in the fruit?