For adding an additional Masonry item type you need to add the following Page-TypoScript:

TCEFORM.tx_masonry_domain_model_masonryitem {
  template {
    addItems {
      LightBox = Light-Box
    }
  }
}

Afterwards create a Fluid-Partial with the same name as the key, for example Partials/Masonry/LightBox.html.