How to theme a form using hook theme into a collapsible table

Hi,
I'm writing a module that displays editable tables, so I'm thinking of using Form API to create a form with all the fields I need and than use hook_theme to format the form as a table.
in the theme_myform() I can access all the fields I've defined and their values, and format them as a table, but I don't know how to add a collapsible div around the table (even if I add a #collapsible tag to the form, when I'm in the theme function I don't know of to use it.
some code from my module:

function theme_myform($form) {
	$rows = array();
	foreach (element_children($form['portfolio']) as $key) {
		$row = array();
		if (isset($form['portfolio'][$key]['name'])) {
			$row[] = array('data' => drupal_render($form['portfolio'][$key]['name']));
			$row[] = array('data' => drupal_render($form['portfolio'][$key]['category']));
			$row[] = array('data' => drupal_render($form['portfolio'][$key]['createdon'])); 
			$rows[] = $row;
		}
	}
 
	//table headers.
	$header = array();
	$header[] = t('Name');
	$header[] = t('Category');
	$header[] = t('Created on');
	
 	$output = theme('table', $header, $rows);
	$output .= drupal_render($form);
	return $output;

The 'portfolio' is a fieldset in the $form that is defined as collapsible, but it gets ignored.
I'm using D6 but I don't mind using D7 if it's easier/better

Any help if welcome,
Thanks