Old question, but maybe my reply can be useful for someone.

The Open XML sdk doesn't allow you to convert to other document formats itself so you will have to use an other option to do this.

The obvious alternative is to use Word itself. Word allows you to save a document as RTF and you can easily automate it using C#.

If you need an option that does not include Word, maybe you need to run server-side, then you can either use the Word Automation Services (but then you need Sharepoint I think) or you can use third-party options like Aspose.