How to use a node field in custom form programmatically.??

I was saying that. there was a method with cck in drupal 6 to attach a cck field in our custom form. like.

$field = content_fields('field_name');  // field_name is cck field (text_field,text_Area,image_field anything.)
$form['#field_info'][$name] = $field; 
$form += content_field_form($form, $form_state, $field); 

So how can i do the same with drupal 7 ?? I had a form and i want to use the field that i created for a node type.

Yes, I checked that but its not helpfull. what i need is something like this.

function drupal_form(){

 $form['field'] = array(
     '#type' => 'textfield',
     '#title' => t('Name'),
);

$form['field_name'] = some_function('existing_field_name'); // "exiting_field_name" is the field that i created on a node_type and this code should return the field structure like form api.so it can be rendered as form element.

$form['submit'] = array( 
   '#type' => 'submit',
   '#value' =>  t('submit'),
)

return $form;

}

Please let me know if its not clear.

I also tried this . but it didn't work as i don't have any "$entity" to pass in function, also the view mode should be form to display it like a form element which is not supported here. i guess..........