is it possible in a custom post, create custom fields as well whitin?

I think you’re looking for add_meta_box

http://codex.wordpress.org/Function_Reference/add_meta_box

// Metabox actions
add_action( 'add_meta_boxes', 'metabox_create' ); // create
add_action( 'save_post', 'metabox_save' ); // save

// Create metabox
function metabox_create() {
  add_meta_box( 'metabox_name', 'Metabox Title', 'metabox_content', 'my_custom_post_type', 'normal', 'high' );
}

// metabox content
function metabox_content() {
  global $post;

  // custom field data
  $custom = get_post_custom( $post->ID );
  $field_name = $custom['field_name'][0];

  // contents
  ?>
  <label for="field_name">Field name</label>
  <input name="field_name" id="field_name" value="<?php echo esc_attr( $field_name ); ?>" />
  <?php
}

// save metabox fields
function metabox_save() {
  global $post;

  // Ignore autosave
  if ( defined( 'DOING_AUTOSAVE' ) && true === DOING_AUTOSAVE ) { return; }

  // Save post meta.
  update_post_meta( $post->ID, 'field_name', $_POST['field_name'] );
}