A common way to get fruit flavours into a beer is to soak it in vodka for a few days and pour it in. I imagine this would work with chopped up pineapple too.
The only thing I can add is that in South Africa they make Pineapple beer which is made by just chucking in pineapple skins , water, sugar and some yeast - I understand that in some cases people don't add yeast as the natural yeasts present on the pineapple will start the fermentation. I made some with bread yeast and it kept its pineapple taste but I wonder if you might introduce "wild" yeasts as well...