Hi Fede,
Hmmm this could be difficult to do in the Custom Field Template by itself. What you are trying to do is to update the_excerpt field on post save. You will need to hook on the save_post to do that.
The reason is that when the custom field saves its data to the post_excerpt field, it is immediately over-written by the actual post_excerpt data.
You will need to write something in your theme's function file that runs on save_post.
add_action('save_post', 'cft_save_advanced_excerpt', 10 , 2);
function cft_save_advanced_excerpt(){
global $post;
// your custom field in the CFT would be "[advanced_excerpt]"
$advanced_field = get_post_meta( $post->ID, 'advanced_excerpt' , true );
$fields = array();
$fields['ID'] = $post->ID;
$fields['post_excerpt'] = $advanced_field';
wp_update_post( $fields );
}
You can also use css in the CFT to hide the post excerpt - or better yet, the Adminimize plugin and hide it (the post excerpt box) all together.
Note: I have not tested any of the above code, but should at least point you in the right direction for more Googling. Good luck!