when sending a Request for Approval, I want to limit the Approvers it can be sent to

when sending a Request for Approval, I want to limit the Approvers it can be sent to

Customer wants that the Requests only be sent for Approval to valid Approvers, not to just any user. How can this be done? Is there an attribute somewhere so that only certain users (i.e. Supervisors) can be selected as Approvers?

thanks -Ricardo