In ~\workarea\ContentDesigner\EkRadEditor.js, search for ".ParagraphsArray". All Formatting dropdown list options are defined in .ParagraphsArray as [Text, Value] pairs.
You can back up the original EkRadEditor.js and modify this array as needed. The localized string in the Text of the array is defined in ~\workarea\Foundation\RadControls\Editor\Localization\*\Main.xml. The id attribute in the XML files matches the first attribute of the .GetLocalizedString function. For example:
me.GetLocalizedString([id], [Default string text])
Therefore, a new row of the array can look like
" + me.GetLocalizedString("sH6", "Heading 6") + "
where the first comma (,) denotes the next row for the array.
The following code snippet adds Heading 6 to the list. In Main.xml, you can add a new row inside for the Heading 6 in English and all other languages