How to add a Capability to a User Role?

You can use WP_Role class,

// get the the role object
$role_object = get_role( $role_name );

// add $cap capability to this role object
$role_object->add_cap( $capability_name );

// remove $cap capability from this role object
$role_object->remove_cap( $capability_name );

So to address your original question about how to enable Admins to enter SCRIPT and IFRAME tags into post content, you’re looking for the ‘unfiltered_html’ capability, which in Multisite is only granted to Super Admins.

// get the the role object
$admin_role = get_role( 'administrator' );
// grant the unfiltered_html capability
$admin_role->add_cap( 'unfiltered_html', true );

or you can run this once in your functions:

/* Roles & Capabilities */
add_role('professional', 'Professional User', array(
    'read' => true, // True allows that capability, False specifically removes it.
    'edit_posts' => true,
    'delete_posts' => true,
    //'edit_published_posts' => true,
    //'publish_posts' => true,
    //'edit_files' => true,
    'upload_files' => true //last in array needs no comma!
));