Theres a good brewing network podcast from a few years ago on adding fruit/syrups to the wort. Can't remember the exact details, but was pretty simple and JZ et al gave a good rundown on the process.
I've added raspberry to a grain and grape FWK beliner weiss in the past with good effect. I made a puree with frozen berries and some boiling water to a consistency of pumpkin soup (sorry, the only thing I could think of). To test the amount, I drew a 200mL sample from the fermenter, and added 2mL (i.e. 1% of the volume) at a time until I got the flavour just beyond what I thought was good (knowing that a reasonable amount of flavour would be lost). The puree went in around 2/3 of the way through fermentation, and I "stirred" it using a sanitised piece of hose and my oxygen stone connected to my CO2 tank.
Ended up a lovely pink colour, with a hint of raspberry flavour and nice sourness.. The final ABV as around 4% I think. I didn't bother trying to calculate the extra sugar and the effect on the final alcohol content (was never going to add much more than about 0.5-1% so wasn't too concerned).
I was concerned about infection, however decided it was a risk worth taking. There was also a risk of O2 exposure (which I attempted to minimise as much as possible), but the beer wasn't going to last more than a couple of weeks in the keg in the middle of a Geelong summer.
The process wasn't perfect, or probably best practice (and it's only something I'd do with a beer designed to be drunk very young), but I was pleased with the results.
Only thing I'd say, is make sure you allow it to ferment out completely if bottling. Put the fruit (or juice) in, check a gravity reading then don't package until you've got stable gravity across 2 different readings (ideally 24-48 hours apart).
JD