You can use Set new design on the Button & NavBar Tool to replace the existing button for the selected state. If the existing button is off-center, Live stretching (also on the Button & NavBar Tool) controls the position of the new design.

For example, if the mouse-off state looks like this:

image\STATE1.gif

and the mouse-over state looks like this (the dotted outline shows the mouse-off state):

image\STATE2.gif

then, with Live stretching selected, Set new design for the mouse-over state positions the new design in the center of the button (that is, in a different position to the original mouse-over design):

image\STATE3.gif

With Live stretching deselected, the new design is in the same position as the original design:

image\STATE4.gif