Update on this question... so far all I've found is the workaround of not using the Category selection at all when I'm entering new websites. You can leave the drop down menu for Category alone and OpenX will still accept the new website entry (though entering an email address each time is required).
I'd still like to know how to alter the built-in list of categories, so I'll probably post a question on the forum at OpenX to see what they say about that.
If anyone knows how to do this, please do post your solution here. And if anyone has some tips or tricks they've learned about OpenX, feel free to add them to this thread too.
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