To add excerpts to Pages, from 3.0 on you just need to add the support for the excerpt
feature for the page
post type with add_post_type_support called in the init
hook.
Admin forms
Adding excerpts to Pages
By the Web Warlock, Wednesday, 28/Jul/2010 3:47
$_wp_post_type_features
By the Web Warlock, Wednesday, 28/Jul/2010 1:16
The global variable $_wp_post_type_features stores the features enabled for the different post types as a boolean value of the index $_wp_post_type_features[post_type][feature].