Thanks, PinkElephant! I'll try to take it to support (although the process to do that with a free account on the Help & Learning website was not very clear to me). At least it's nice to know there's not something I'm missing--the feature simply isn't there.
Also, cswilly, I'm using a Mac, so I'll have to go digging around for the file location of "Custom Dictionary.txt." Even so, I'm not sure I'm brave enough to attempt to edit it if it's locked. But thanks for the input!