how to create path node/{some id}/add/{node_type} which loads node/add/{node_type} page
We can achieve this by creating route as below.

cust_group.route_name:
  path: '/node/{group_id}/add/{node_type}'
  defaults:
    _controller: '\Drupal\cust_group\Controller\CustNodeController:add'
  requirements:
    _permission: 'access content'   ( change permission according to your requirement)

And create a CustNodeController controller with method. 

/**
 * Returns responses for Node routes.
 */
class CustNodeController extends ControllerBase {

  /**
   * Provides the node submission form.
   *
   * @param \Drupal\node\NodeTypeInterface $node_type
   *   The node type entity for the node.
   *
   * @return array
   *   A node submission form.
   */
  public function add($group_id, NodeTypeInterface $node_type) {
    $group_id = 

    $node = $this-&gt;entityManager()-&gt;getStorage('node')-&gt;create(array(
      'type' =&gt; $node_type-&gt;id(),
    ));

    $form = $this-&gt;entityFormBuilder()-&gt;getForm($node);

    return $form;
  }


}


make sure that {some id} != {node}.