Is there any integration of the Entity registration module with modal forms?
Few alternative way may helps you.

Entity popup module.


  This module allows you to show an entity in a popup. For flexibility,
  it doesn't provide any popup plugins itself, but it provides a menu
  callback that allows you to load the entity into any popup you desire.


NG Lightbox module:


  Lightbox any Drupal page such as the login page or comment reply.
      Works with forms out of the box, including webforms, node edit forms and more.
      Lightweight, only 1.2kbs in D7. D8 uses cores modal.
      Default responsive theme for the lightbox that can be customised.