You can add new TabSets easily:
function getCMSFields() {
$fields = parent::getCMSFields();
$fields->addFieldToTab("Root.Content.ContentBlocks",new TabSet('ContentBlockSet',
new Tab('HeaderContent', new HTMLEditorField("HeaderContent")),
new Tab('LeftContent', new HTMLEditorField("LeftContent")),
new Tab('RightContent', new HTMLEditorField("RightContent"))
));
$fields->addFieldToTab("Root.Content.Widgets",new TabSet('WidgetsSet',
new Tab('WidgetsLeftTop', new WidgetAreaEditor("WidgetsLeftTopArea")),
new Tab('WidgetsLeftBottom', new WidgetAreaEditor("WidgetsLeftBottomArea")),
new Tab('WidgetsRightTop', new WidgetAreaEditor("WidgetsRightTopArea")),
new Tab('WidgetsRightBottom', new WidgetAreaEditor("WidgetsRightBottomArea"))
));
return $fields;
}
http://doc.silverstripe.org/tabset