When I would try this I would buy frozen, unsweetened fruit (natural sugars are there but no added sugar), let them thaw and crush them as best I could and then I think I would bring the mush to 170° or so in a pot to sanitize it as best as possible. There was some discussion about not bringing the temp higher because it would "set the pectin" and cause some problems with clarity, etc. I'm pulling this out of long-term memory so if I have misspoken, please correct me. Then I would add that to a secondary and transfer the finished beer on top, leave it for a week or two and then bottle or keg. But in doing that, the flat, finished beer would kick up a second fermentation that seemed to go on FOREVER. There would be enough yeast in the beer to start another fermentation. That lengthy process ended up drying out the beer. Also, I do not use secondaries anymore and I threw all of mine out. So I would have to add the fruit to the main fermenter, let it do its thing and then transfer directly to a keg. I feel like there are so many caveats to fresh or frozen (or purees) that I always come back around to extracts. I remember a brewer saying that he would make a wheat beer (50/50 2-row, wheat, one hop addition) and then transfer it to the keg after dropping TWO small tubs of Crystal Light lemonade powder into the keg. He said it provided the perfect lemon character in the wheat beer. I know some of you are wincing right now and I get it. But sometimes you see the finish line and there are a number of ways to get there. I believe I tried using unsweetened strawberry drink mix one time to get strawberry flavor into a wheat ale. The result was questionable but it was also a long time ago and I might do it differently now. The point is that there are various ways to achieve this. I saw sugar-free raspberry and strawberry syrups at my local grocery store thinking that might be a way to go. I didn't try it but you never know.
Another loosely-related tangent: If you're going to have to add A LOT of fruit to achieve the flavor you want, you're going to run into a volume issue. You might have to brew only 4 gallons of beer because the rest of the volume might be made up of fruit depending on your vessels and your type of packaging. Seems like another nod to extracts... you don't have that problem.