Yep set your first key frame as the ball touches down. For example, in a frame of 30 with a bounce at around 15, set the first frame at 14 with squash at zero. At 15 set another key frame with the squash starting. At say 18 set on for full squash, at 19 set a new one to began reversing the squash. Around 21 or 22 set another key with the squash at zero as ball rebounds. You get the idea. You need to proceed any key changes in motion with key frames otherwise Poser sometimes does weird things.