It is fine to add adjuncts to your beer in the fermenter.

Many homebrewers use home or commercial pureed fruit products when adding it to the fermenter rather than earlier in the process, to avoid blocked valves and to reduce the risk of contamination since many pureed fruits are flash-pasteurised. 

If you prefer to add whole fruit to the fermenter, we suggest mashing or blending the fruit to release the juices and create more surface area for the fruit to interact with your beer. Also, using a muslin bag (hop bag) for your fruit will also help reduce the risk of blocked valves in your fermenter. You may want to freeze and then thaw the fruit a few times to break down the cell walls helping the extraction of fruit flavour and sugars.

You can pasteurise the fruit at 76°C (170°F) for 15 minutes to ensure that any pathogens on the fruit are neutralised before adding to your beer, or skip this step and hope for the best!

Another possible step to take when adding whole fruit to your beer would be to freeze it. The cell walls will begin to degrade - and the fruit will impart a much stronger flavour on your beer.

The following chart from the May/June 2015 issue of Zymurgy magazine can be used to help you determine an appropriate amount of fruit to add to your beer.