Do you mean depth-of-field? That's generally very difficult and tedious to do in software and Premiere isn't really equipped for it. That background out of focus look is best achieved in-camera by zooming all the way in and opening up the aparture to max.
you can use dynamic link to open a new ppro linked project in AE and do some rotoscoping on your subjects and use a lense blur on the duplicate background plate, .....ooooorrrrrr you can do as suggested above to get the most out of your cam as far as shallow DOF .. or go for a 35mm adaptor. personally id go for the second 2 any day.
in a pinch ive done a bit of roto to throw the backgroud more the defocused direction, but it is a pain like no other pain pending if your subject matter is moving and what the shutter speed was. because rotoing motion is less easily convincing when there is motion blur involved.