Not as normal part of the system but with UDFs, scripts and the add-on Sage Alerts and WorkFlow you can design a system to do this.

Indeed, I built this kind of thing once (but the solution never went Live... client changed their mind). 

New PO as Master only.  Standard PO only allowed from an approved Master.  Scripts control all that... with alerts to email different potential approvers depending on the Master PO specifics.  Allowed approval levels (header checkboxes) controlled by Roles, enforced by scripts.  Form modified to print no details for unapproved Master orders.

A lot of work, but possible.

A formal / professional solution (like Jeff's suggestions below) would certainly be more polished / full featured, but with a bigger price tag too.

If you are talking about approvals at the payable step (not the PO step) that is a completely different story.

Indeed, I built this kind of thing once (but the solution never went Live... client changed their mind). 

New PO as Master only.  Standard PO only allowed from an approved Master.  Scripts control all that... with alerts to email different potential approvers depending on the Master PO specifics.  Allowed approval levels (header checkboxes) controlled by Roles, enforced by scripts.  Form modified to print no details for unapproved Master orders.

A lot of work, but possible.

A formal / professional solution (like Jeff's suggestions below) would certainly be more polished / full featured, but with a bigger price tag too.

If you are talking about approvals at the payable step (not the PO step) that is a completely different story.