How to add custom function to pluggable.php

You should not and do not need to write to pluggable.php (or any other core WordPress file).

You can override a pluggable function simply by defining it in your plug-in’s files (or for themes, functions.php). These get loaded before pluggable.php, and so the function definition given in your plug-in file is used in preference to that in pluggable.php.

You’ll want to wrap you function definition inside a function_exists conditional:

if( !function_exists('function_name') ){
   function function_name(){
      return 'Hello World';
   }
}