I looked around to see how to add words to GoogleSpell, and ended up nowhere. This is not really that surprising, that Google would want to control its own dictionary. Somehow, it would be nice if Moodle could use GoogleSpell, plus a local-defined dictionary. Maybe a simple text file with words. The logic would be 1) check GoogleSpell, and the 2) check the "system" dictionary. Of course, I have no ideas whether what I suggest could ever really be implemented. I am just dreaming a little.
(I would immediately add "Moodle" to this dictionary, and also "GoogleSpell".)