You will need to edit your front-page template and before (or after) your loop, you need to make a call to
get_post() so load your custom page. See the documentation for details.
You can then render your customisable page on the homepage.
For example, if your special page ID is 42:
<?php $custom_page = get_post(42); echo apply_filters('the_content', $custom_page->post_content); ?>