JHtmlSidebar is pretty simple and doesn't support headings/separators or anything like that. But you can fake them by adding the required HTML to the language string used as a name for the entry and omitting the link.
The language string:
You will firstly need to create a helper class that extends ContentHelper, like so:
class MyComponentHelper extends ContentHelper
protected $config = null;
public function __construct($config)
$this->config = new ...