I don't think the exporter does DoF, you have to add DoF settings to the .lxs file, like so:

Camera "perspective"

 "float fov" [14.8586407274]

  "bool autofocus" ["false"]

 "float lensradius" [0.00625]

 "float focaldistance" [4.0]

lensradius is dependant on the camera aperture setting, this is for an Fstop of 2.8, lower Fstop is larger radius.
focaldistance is distance in meters to the focal point.
autofocus must be false.

 

Software: OS X 10.8 - Poser Pro 2012 SR2 - Luxrender 1.0RC3 - Pose2Lux
Hardware: iMac - 3.06 GHz Core2Duo - 12 GB RAM - ATI Radeon HD 4670 - 256 MB