Susan,

I do not know whether it is possible. Seriously, though, you do not want this!

1. It will slow Word down.
2. It will beep when you know to correct before it beeps.
3. It will drive your users nuts.

Word has the red squiggly lines which should be sufficient. You can also press F7 to run a spell check on the document and go through perceived errors one at a time.