I just ran across two inline style converters in a Smashing Mag article.
These are great tools; Inline styler is especially nice for allowing you to feed it a URL rather than copying and pasting CSS styles and HTML code into separate boxes. The reporting on compatibility issues is also helpful.
If you aren't familiar with the joys of HTML and CSS for mass e-mail, external stylesheets and styles in the head section are just plain troublesome in e-mail clients, since they're often ignored. Inline styles are the ticket. Other than pseudo-classes (e.g., a:hover) everything should be inline and inheritance should be leveraged.