sorry, but you cant do it in FCP.
you'll just have to dive and and move the keyframes you need to move.
wont take too long.
another approach to dipping music under clips that may well end up moving is to use a really old method:
blade the music clip under the voice,
dip the sections under voice,
and apply transitions
if you have to change things around, you just roll the edit point,
so two big moves per change rather than four little ones.
nick