Coping data between metadata fields is easy via AppleScript and VB Script. Only use the script on several hundred images at a time as xMedia can choke on metadata scripts when dealing with over a thousand images.
Have a look on the old iView forums
for a script. If there isn't one specifically for Object Name to Description find a script that is close, open it in TextEdit or Notepad and modify it. You''ll need to change any mention in the script of "iView MediaPro" to "Expression Media".