I run a Blues Brothers forum, and I needed a way to add "Aykroyd" to my dictionary (somehow "Belushi" was already in there!), as well as any words we come across that should be in the list.
I searched this thread, and the other vB Spell thread and couldn't find any easy way to add new words, apart from the Learn cookie method (which isn't much help in this case). Until this feature is added (perhaps as an AdminCP thing), here's a page I threw together quickly to generate the SQL required to add a new word to your dictionary. I assume this also works for 3.5.x, if they're using the same vbspell table structure.
http://www.bbhosts.net/vbspell/
You should be able to just copy/paste the SQL into phpMyAdmin, or whatever database admin software you use.
If it's useful to anyone let me know, extra feature ideas are welcome