The style sheets aren't used to populate the style list. They are only used to define the css that is applied to the html editor body when displaying content.
Styles need to be added individually. We considered adding a feature to get styles from a style sheet, but because of the complexity of css styles and the lack of important details like the tag to use, label to use, and the ability to define nested styles and classes as well as being able to specify inline styles it seemed better to have the GUI for adding styles individually.
Are you trying to add a whole lot of styles to this list? Do you have a sample css you are trying to get styles from I can look at? We could consider adding an import from css feature in the future, but it doesn't work that way in this release.