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Swype makes it pretty easy to add words to its auto correction database -- too easy for me: mine now has all of my common mispellings.
It's fairly easy to do in SlideIt as well, but in a compound-heavy language, it just isn't worth it. The main problem is that it insists on inserting a space between words, even when none is called for. There's actually a setting for this -- "automatically insert space" -- but either it works differently or not at all. Perhaps I should just file a bug report.