Attached Link: http://www.kaydara.com/downloads/tutorialdemos/MotionBuilder_Poser.pdf

You mentioned that you have Motionbuilder 5. At the attached link is a tutorial (nudity in the tutorial images) that provides instructions for exporting the rigging for any model in Poser into Motionbuilder. You can then export a Motionbuilder animation to a bvh file that is specific to your particular Poser figure. Works great!