Another Python script is included with Pro Pack; look in the Utility folder for "dropFigToFloorAllFrames.py". This works exactly like the Figure Menu --> Drop to Floor command, but it's applied to all of your animation frames simultaneously.

Unfortunately, this script isn't always the best choice; for instance, if you have a character jumping and skipping about, it'll leave the figure grounded.

You can also try adjusting the Body's y-trans values to keep the figure at ground level.