Something

function smarty_function_createArray($params, &$smarty)
{
	if(!empty($params['_assign'])) {
		$assign = $params['_assign'];
		unset($params['_assign']);
	}
	if(isset($params['_merge']) && is_array($params['_merge'])) {
		$params = array_merge($params, $params['_merge']);
		unset($params['_merge']);
	}
	if(isset($assign)) {
		$smarty->assign($assign, $params);
	} else {
		return $params;
	}
}

{createArray _assign='myarray' item1='Item #1' item2='Item #2'}
{$myarray.item1}