There is no way to "convert" them as far as clicking a button and choosing "Conver to CSS" or anything similar.
You could create a new external .css file and slowly write rules in css to replace html attributes then delete those attributes from the code of the page, but you're best bet will be to rebuild the page from the ground up using css.
Fact is, you might as well just redo the site.
And Dreamweaver has a number of fairly simple layouts you can use to do that, all with a separate CSS sheet. And you can get free CSS layouts almost anywhere on the Internet these days (better ones are for sale, still better ones are available as Dreamweaver Extensions).
Look at your site. Figure out how much it would cost to redo it from scratch. Figure out how much you would save by purchasing someone's design (if it suits you). Then use this maxim:
Good, Fast, Cheap. Pick any two.