Thanks a lot for your help. This is certainly a simple and good way.
Especially the URL to hunspell was very helpful, because it convinced me, that the word list is as simple as it can be and requires no magical syntax.
I think there's one caveat to it: When I add a new word to the dictionary, it will get added to the default dictionary and not the custom one. So one would need to add it there, too. This works but is not the most elegant solution I can think of.