If a person being assessed is going to choose their own 360 feedback providers, then you can add an approval step. With an approval step, the list of feedback providers will be sent to the approver before invitations for feedback are sent out.
The approver can then review the list and make changes if needed. When the approver is happy, they can approve the list and the invitations asking for feedback will automatically be sent out.
Each person being assessed can have a different approver if needed.
The easiest way is to set the approvers when using the Excel bulk upload.