Ive got a blackberry witbier in the keg right now conditioning....I put in half the can of puree at fermentation and then used sorbate and added the last half the can of puree in the keg.
Sorbate was used so the yeast wouldn't ferment anything after the primary fermentation was complete.
It tastes pretty good, but a sharp tartness with the blackberry, its been in the keg for a month and has been fading nicely...should be ready to drink soon.
now if you using the concentrated flavoring add that at bottling/kegging time
hope this gives some info