Just code a plugin to add the button, something like this should work:
class button extends rcube_plugin {
function init() {
$this->add_button(array(
'name' => 'the button',
'class' => 'button-class',
'label' => 'Google',
'href' => 'http://google.com',
'target' => '_blank',
), 'taskbar');
}
}