In order to include external style sheets, you can use the setStyleSheetURL function, e.g.:

...
eop.setStyleSheetURL("http://myserver.com/styles.css");
...

(see also the edit-on Pro (Version 4.2) Integration Manual chapter 2.2. JavaScript API / Java API)

Style classes included in the style sheet are displayed in the advanced style sheet combo box, which can be specified via ADVANCEDSTYLESHEET action within the uiconfig.xml file, e.g.:

...
<toolbar>
<choice actionid="ADVANCEDSTYLESHEET" />
...
</toolbar>
...

(see also chapter 3.2. Actions)

Then you can select all defined style sheets from the combo box.
Please note: Styles loaded from an external stylesheet can only be applied to the content, but not edited within the editor. If you want your users to be able to edit style definitions, you must insert the stylesheet definitions directly into edit-on Pro, e.g. via setStyleSheet() JS API function.
(see chapter 2.2. JavaScript API / Java API)