First of all, I don't want to sign up for any kind of mailing list just so that I can contact someone to say, "I found a word in the Open Office Writer Thesaurus that is not a word".
That said, "transgendered" is not a word. Many people who identify as transgender would be offended by being called "transgendered" as it implies that being transgender is something that they have done as opposed to an inherent identity. Therefore, I believe that the entry in the Open Office Writer thesaurus for "transgendered" should be changed to "transgender"