I brewed an Irish stout and racked it to secondary on top of 3 cans of oregon fruit's bing cherries. I drained the syrup which in hindsight I should have added to enhance the flavor. The fermentation took off again after adding the fruit. I let it sit for 10 days, it should have sat for longer to get more flavor though.
I also did an American wheat on top of Blackberries. This time I added the syrup and it helped the flavor tremendously. Fermentation took off so hard in the secondary it blew the airlock off. I assume its from the yeast eating up the sugars from the syrup. I hooked up a blow off tube and let it go for a week, then cold crash to settle the fruit and then siphoned it off through a piece of cheese cloth to catch the solids. it turned out very good
I followed the advice of others to add the fruit in the seconday, although Im sure it would work at the end of the primary.