I'm trying to add a new style to "--Styles--" TinyMCE:

I go to "sapphire/forms/HtmlEditorField.php" and up-commented

new HtmlEditorField_dropdown("mceSetCSSClass", "styleSelect", array(
"mceContentBody" => "mceContentBody",
))
inside the function Buttons.

I also added span to "--Format--":

new HtmlEditorField_dropdown("FormatBlock", "formatSelect", array(
"<p>" => _t('HtmlEditorField.FORMATP', "Paragraph", PR_MEDIUM, '<p> tag'),
"<h1>" => _t('HtmlEditorField.FORMATH1', "Heading-1", PR_MEDIUM, '<h1> tag'),
"<h2>" => _t('HtmlEditorField.FORMATH2', "Heading 2", PR_MEDIUM, '<h2> tag'),
"<h3>" => _t('HtmlEditorField.FORMATH3', "Heading 3", PR_MEDIUM, '<h3> tag'),
"<h4>" => _t('HtmlEditorField.FORMATH4', "Heading 4", PR_MEDIUM, '<h4> tag'),
"<h5>" => _t('HtmlEditorField.FORMATH5', "Heading 5", PR_MEDIUM, '<h5> tag'),
"<h6>" => _t('HtmlEditorField.FORMATH6', "Heading 6", PR_MEDIUM, '<h6> tag'),
"<address>" => _t('HtmlEditorField.FORMATADDR', "Address", PR_MEDIUM, '<address> tag'),
"<pre>" => _t('HtmlEditorField.FORMATPRE', "Preformatted", PR_MEDIUM, '<pre> tag'),
"<span>" => _t('HtmlEditorField.FORMATPRE', "parentItem for Quickinks", PR_MEDIUM, '<span> tag'),

)),

But nothing is changed inside my TinyMCE

Read this:
http://www.silverstripe.org/customising-the-cms/show/276090?showPost=277432

Try clearing the browser cache!

From what I found out:

to add classes to the dropdown box, you don't need to change anything in "sapphire/forms/HtmlEditorField.php". You need to add class "editor.css" to your theme (css folder) and define classes there. All the classes from editor.css will automatically appear in the dropdown menu of TinyMCE.

And how to add a new tag in the drop-down box......I still don't know