In BeOS 5, BeMail grows some new features, including the ability to spell-check email on the fly. While typing a new message, tap Alt+; (alt semicolon). All potentially misspelled words will turn red. Right-click on any red word and a list of possible alternates will appear in a context menu. If you don’t find the alternate you’re looking for, select Add.
That’s fine for adding one word at a time, but what if you want to add or remove a batch of words to your dictionary all at once? Just open /boot/beos/etc/word_dictionary/user in a text editor and make your changes. To modify existing dictionaries, edit any or all of:

/boot/beos/etc/dictionary
/boot/beos/etc/word_dictionary/geekspeak
/boot/beos/etc/word_dictionary/words