Adding Custom Capability Based on a Field

I'm using Formidable Pro with User Registration. Also, using S2member where I have two user roles "Applicant" and "Partner." I use custom capabilities to determine different parts of pages a partner can see (based on product, location, etc).

I'd would like to have other members of an organization that has already been approved to be a partner register using a special code given to them, so they can access the right pages automatically without having to be manually approved and have the capability added manually. This special code would translae to the custom capabilities.

Basically I'd like to do something like this:

add_filter('frmreg_new_role', 'frmreg_new_role', 10, 2);
function frmreg_new_role($role, $atts){
  extract($atts);
  if($form->id == 5){
    if($_POST['item_meta'][125] == 'Author') //change 125 to the ID of your field and 'Author' to the value of your radio button
      $role = 'author';
  }
  return $role;
}

But instead of assigning the role, assigning the custom capability..  $user = wp_get_current_user(); $user->add_cap('access_s2member_ccap_music'); 

Can anyone help?