How to replace node_form() with my custom form callback?
Variant 1. You can use hook_menu_alter and you can set your callback

/**
 * Implements hook_menu_alter.
 */
function MYMODULE_menu_alter(&amp;$items) {
  $items['node/add/superpage']['page callback'] = 'MYMODULE_superpage_form';
}


Variant 2. You can use hook_form_FORM_ID_alter  and override/edit the form

/**
 * Implements hook_form_FORM_ID_alter()
 */
function MYMODULE_form_superpage_node_form_alter(&amp;$form, &amp;$form_state, $form_id){
  // your code
}