The upper image is the BVH as it was imported.  The lower image has three keyframes one on either edge of the layer and the center one is the one I draged down to pull the entire range down.  It is not like yours where you dropped all the same amount mine slowly ramps down peaks at the keyframe and slowly ramps back up.  No sudden change of position.  Not hard.

Justs takes a little time to find out what your tools can do.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13