You have several options and all have varying level of complexity.

You could use and AEXQUERY macro to evaluate whether a person is a manager assuming you have that information in your AMS system with 'reports to.'

You could also write a validation rule that only allows certain people to save with a specific category, but this can be troublesome if the list of managers is large.

You could also add a boolean field to the contact record to denote if someone is a manager in helpdesk and set up validation on that field.  

Any route you take will involve a bit of work to get you where you need to be.