How to add custom function to pluggable.php

Question

I am working on a WP plugin and I want to add a custom function to pluggable.php (located in /wp-includes). I am calling that function from admin.php (located in /wp-admin)

eg. Consider the function auth_redirect which is called from admin.php. auth_redirect is the function defined in pluggable.php which checks for logged users, else it redirects them to login page.

Similarly, I have my own custom function.

So, is there any specific hook or filter which I must use to attach my function to pluggable.php.

Currently, I am using fwrite() to add my custom function to pluggable.php. But, I guess this isn't the right way.

So, can you suggest me the right way of inserting my custom function and calling it from admin.php

Any help will be appreciated.