Currently, if you have the feature "Ability to create purchase order request in POS" enabled, any worker who can receipt a purchase order on Store Commerce can add un-planned items to that order. This is unless you activate change management for POs, and then no one can add un-planned items to the POs. This is very binary and perhaps a more controlled approach could be as below.

1. Have a permission setting in the POS permission groups to allow/prevent adding un-planned items to POs on POS.
2. Restrict the addition of un-planned product to only where that PO vendor is an approved vendor for that product.
3. Have the addition of an un-planned product to a PO on POS trigger a purchase order line workflow to allow the appropriate group to accept/reject the addition to the PO.