Can employees who have not been identified as affected or surplus step forward and volunteer to depart?

Yes, if the department in question makes this option available.  Departments are considering various ways to reduce the impact of the required reductions on its current employees, which may include inviting employees to express their interest in volunteering to be laid-off.  It is at the discretion of management to accept an expression of interest to “volunteer”.  Accepted volunteers could then be entitled to the appropriate Workforce Adjustment (WFA) provisions whether they have received an affected status letter or not.