See the workaround in my thread "Table creation and changing settings with hidden table inspector" in OmniGraffle Extra Forum.
http://forums.omnigroup.com/showthread.php?t=12952
This should work with tables in Word documents, too. If that won't work with the table on the whole, so only select the table
cells. Sometimes that seems to make a difference.
So I hope that solution may help you.
Rainer