While adding fruit directly to the secondary fermenter may an easy way to get it into the fermenter, it makes it difficult to separate it from the beer later. The times that I have done it this way my bottles have always ended up with some pieces of fruit in them. While the fruit chunks are not any sort of hindrance to the flavor, they are certainly not appealing to have floating around in your glass.
Frozen fruit is an economical choice, and has additional benefits to fresh fruit as well. The freezing processes breaks down the cell walls allowing more of the fruit flavors into your beer. In addition, contamination is less of an issue because cold temperatures inhibit bacteriological growth. Although some people add frozen fruit directly to the fermenter I wouldn't recommend it for two reasons. First, the cellulose of the fruit will release the juice into the beer, but it will be replaced by beer in the processes. I have had as much as 33% trub loss because of this issue. Second, floaters.
The best way I have found to add fruit to your beer is to pasteurize the fruit. Not only does this processes remove almost all of the cellulose, but it will also kill any bacteria that may have somehow made it to this point.
1) Add the frozen fruit to a sauce pan.
2) 1 pint of water for every pound of fruit.
3) Apply medium high heat to the sauce pan stirring constantly and mashing. (A potato masher works great for this)
4) Once the temperature has reached 160 degree turn off the heat.
5) Pour this mixture through a sanitized mesh strainer and into a second sanitized container and affix the lid.
6) Allow this to cool for about 20 minutes and then move it to the refrigerator.
7) After several hours in the refrigerator pour the contents into the fermentor.
This is more work on the front end, but it will save you work on the back end trying to separate beer from fruit. And, more importantly, it will yield a few more bottles of beer!