You won't be able to change the format and have it still work.
See posts #4 and #5 of this other thread:
http://forums.macosxhints.com/showth...threadid=10188

A radical expansion of Word’s spell check
 

.
MS Word’s spell check is woefully inadequate. As a copywriter and translator, this is one of my great frustrations with this app.

OK, what I would like to do is a bit more comprehensive: I would like to add every single word from the Oxford Dictionary to Word’s word list. (You know, the excellent free dictionary that ships with OSX.) Naturally, I have no desire to do this manually, one word at a time.

Anyone have a brilliant solution as to how we can do that?

(To put it another way: I want to use Oxford for my spell check, not the native MS Word list...)