How to Add Placeholders

Hi can you please help me how can i add placeholders without touching the core plugin files?

Note: i manage to add one however i edited the plugin php file. i tried to filter the function but i didnt succeed.

            $replace             = array(
                $user_val[ 'profession' ][ 0 ], 
                 $user_val[ 'nickname' ][ 0 ],
                $user_val[ 'first_name' ][ 0 ],
                $user_val[ 'last_name' ][ 0 ],
                get_option( 'blogname' ),
                $wau_too[ $j ],
                $mail_to,
                $unsubscribe_link_ht,
                $subscribe_link_ht 
            );
            $find                = array(
                 'profession',
                 '[[user-nickname]]',
                '[[first-name]]',
                '[[last-name]]',
                '[[site-title]]',
                '[[display-name]]',
                '[[user-email]]',
                '[[unsubscribe-link]]',
                '[[subscribe-link]]' 
            );