Great question and an interesting one both, because it turns out that the Mac OS X spelling subsystem that underlies its common spelling capability is eminently hackable. How can you tell? Simply open up the LocalDictionary file that you reference.

Before I do, however, I will say that it's important to use the right editor: if you use an editor that adds the wrong line-end sequences, you can presumably mess up the dictionary. My suggestion, since I'm comfortable with the command line, is to open up Terminal.app and use either "emacs" or "vi" to add content to the file. Save the original file before you start, though, and presumably you could also just try something else like BBEdit, TextEdit or even Microsoft Word. It messes up? Just copy back the original!

Even if you don't want to hack the file, you can still open it up and have a look by using TextEdit. Launch the program, then navigate to Library > Spelling and open "LocalDictionary".

Here's what mine contains:

Does it work? To test it while in TextEdit, I added the nonsense word "ptoi" and saved the file, then opened a new TextEdit window and typed that word in to the window. The result was that it was still marked as a spelling error. Hmm...

To test further, I logged out and logged in again. The result:

No red squiggly line means that "ptoi" is now a valid word with an understood spelling.

Sweet! So there ya go. You don't need to use Terminal.app or the command line, you simply need to open up the file with TextEdit and add the words you need, save it, and you're good to go. Nice job making this easy to work with, Apple!