add this in a plugin or functions.php and the elements will show up in tinymce's char map. Good special char reference: https://dev.w3.org/html5/html-author/charref

wordpress-add-special-chars-tiny-mce.php

<?php
add_filter( 'tiny_mce_before_init', 'tinymce_add_chars' );
function tinymce_add_chars( $settings ) {
    $new_chars = json_encode( array(
        array( '8224', 'Dagger' ),
        array( '337', 'Double Accute o - lowercase' )
    ) );
    $settings['charmap_append'] = $new_chars;
    return $settings;
}