My own header and footer in login, activate, login

In this 3 files:

wp-activate.php
wp-signup.php
wp-login.php

I'm commenting out all files/actions getting the header and footer from WP. Instead I'm putting in an include tag to include my own header and footer from my own website, since I'm not using the default blog as a front site.

So, the quotestion is: instead of changing in the core files, can I use a plugin or some action to do that?