dynamic-list-options-add.php

<?php

/**
 * Update list options during form render.
 *
 * @param array $options [ [ label, value, calc, (bool) selected, etc ], ... ]
 * @param array $settings
 */
add_filter( 'ninja_forms_render_options', function( $options, $settings ) {
  $options[] = [
    'label' => 'New Option',
    'value' => 'new',
    'calc' => 0,
    'selected' => true
  ];

  return $options;
}, 10, 2 );

dynamic-list-options-before-after.php

<?php

/**
 * Update list options during form render.
 *
 * @param array $options [ [ label, value, calc, (bool) selected, etc ], ... ]
 * @param array $settings
 */
add_filter( 'ninja_forms_render_options', function( $options, $settings ) {
  $options_before[] = [
    'label' => 'Before',
    'value' => 'before',
    'calc' => 0,
    'selected' => true
  ];
  $options_after[] = [
    'label' => 'After',
    'value' => 'after',
    'calc' => 0,
    'selected' => false
  ];
  return array_merge( $options_before, $options, $options_after );
}, 10, 2 );

dynamic-list-options-listcountry.php

<?php

/**
 * Update country list options during form render.
 *
 * @priority >10 After the Country List is populated.
 *
 * @param array $options [ [ label, value, calc, (bool) selected, etc ], ... ]
 * @param array $settings
 */
add_filter( 'ninja_forms_render_options_listcountry', function( $options, $settings ) {
  return $options;
}, 11, 2 );

dynamic-list-options-selected.php

<?php

/**
 * Update list options during form render.
 *
 * @param array $options [ [ label, value, calc, (bool) selected, etc ], ... ]
 * @param array $settings
 */
add_filter( 'ninja_forms_render_options', function( $options, $settings ) {

  foreach( $options as &$option ) {
    if( 'two' == $option[ 'value' ] ){
      $option[ 'selected' ] = true;
    } else {
      $option[ 'selected' ] = false;
    }
  }

  return $options;
}, 10, 2 );

dynamic-list-options.php

<?php

/**
 * Update list options during form render.
 *
 * @param array $options [ [ label, value, calc, (bool) selected, etc ], ... ]
 * @param array $settings
 */
add_filter( 'ninja_forms_render_options', function( $options, $settings ) {
  return $options;
}, 10, 2 );

Added options aren't being submitted, not even being recorded in the submission.