A change planned in a future version will make this very easy.

For now, there isn't an easy way to do it in Simon. If you are comfortable with editing plists, you can certainly do that. Look in the Tests.simon file within the Simon data folder (i.e. "~/Library/Application Support/Dejal/Simon/Tests.simon"). This file is just a plist; you can edit it with your favorite plist editor. (Of course, make sure you quit Simon first.)