While that's possible, well--it would depend on the spellchecker.
There's a spellchecker in my Word 2000 that is very good--good enough that if I am not certain of a word when I am typing in a forum, I open Word and type it there first to see if it gives me an alternative. However, there are words that I confirm on Dictionary.com which are not found in the Word spellchecker, and there are words that I use in my daily work that are not found in it.
Indeed, it does have the ability to add words to it. However, for several documents I have written over the years, I have overflowed the limits on added words. Admittedly, this particularly happens when I'm writing my commentary studies, because the number of Greek words and the number of different forms thereof chokes the spellchecker pretty quickly. It ultimately shuts down and does not do an automatic check anymore; I spot check words for which I am not certain.
The spellchecker in my e-mail program gives me a lot more trouble. It questions common names, hyphenated words, and a lot more. On this site some of the "common" words we use include Cthulu, Alyria, Deadlands, Shadowrun, Magicked, Rolemaster, Dweomer, Sawtooth, and of course Multiverser, not to mention Gygax and Arneson--the tip of a vast iceberg, and I doubt I could get through a day without having to add some word to the spellchecker just for my own posts.
It would be easier for me not to use it. That doesn't mean the rest of you can't use such a thing (although I note that it won't help those who type "dose" for "does" or "were" for "where" or "hart" for "heart"), but it does mean that asking the moderator to update it is probably not the best choice--particularly since I note that the other moderator is currently attending law school and his posts and other written messages are impeccable, so he probably wouldn't waste time on a spellchecker either.
--M. J. Young