As far as I am aware, this is not possible in the Flash Player due to the limited number of HTML tags and attributes supported by Adobe Flash.
You could use HTML formatting in a caption (as documented in this FAQ) such as the following:

<caption><![CDATA[<span style="letter-spacing: 3px;">Caption text goes here.</span>]]></caption>

... but the 'style' attribute would be ignored by the Flash Player and used only by the Mobile Player.