Simple resampling function can rebuild key frame. computed frame which near to key frame is unnatural, and get smooth if use this function. you should try this before exporting BVH file. original key frame is lost, you don't overwrite working file that applied this function.
top menu [Animation] Resample Key Frames, open dialog
(1) Make key frame every  frames
(2) Frame Range :  to [end frame] and click button [OK]
(1), smooth so that value is high. (2), frame range to apply. this function has bug, not refresh animation palette. refresh it, select frame-1 all joints and push button (Spline Section).