Currently RadEditor does not support ShortCuts when its toolbar is set to RibbonBar.

Possible option is to register the shortcut via client-side approach, by using the ShortcutManager object (http://docs.telerik.com/devtools/aspnet-ajax/controls/editor/client-side-programming/shortcutmanager-object):

<telerik:RadEditor ID="RadEditor1" runat="server" ToolbarMode="RibbonBar" OnClientLoad="OnClientLoad">
</telerik:RadEditor>

<script>
    function OnClientLoad(editor, args) {
        editor.get_shortCutManager().addShortCut("TemplateManager", "CTRL+ALT+S");
    }
</script>