Hello Rene,
The only way I know of to add plugins, for now atleast, is to go into moodle root/lib/editor/tinymce/lib.php. At line 127 'plugins' => .... add your plugin name at the end
Then at line 136 add 'theme_advanced_buttons3_add' => "table,|,code,spellchecker | yourpluginname",
Sorry, I don't know enough about upgrading TinyMCE to a newer version in Moodle.
Hope this helps.
Patrick