To add own email tag add following code in your theme’s (child theme’s if any) functions.php

add_filter( 'fep_eb_email_legends', function( $legends, $mgs, $user_email ){
    $legends['custom_tag'] = array(
        'description' => __('Description of this custom tag', 'front-end-pm'),
        'where' => array( 'newmessage', 'reply' ), //where this tag will be used
        'replace_with' => 'this will be sent in email instead of {{custom_tag}}'
    );
    return $legends;
}, 10, 3);