Till ITSM 8.1, when we add any ad hoc approver even if change is not in Active Approval phase, approval is generated and approver even approves it via email(8.1 email based approval).
When he approves it the change is not even in Approval phase.
Consider we have approval mapping for Implementation phase which begins from Scheduled For Approval and goes into Scheduled status after approval.
Now change coordinator wants to add ad hoc for implementation phase.
1) So cooordinator adds that approver at Scheduled For Review status (Non approval phase). So approval will be generated at the same status and approver will approve it and change will remain in Scheduled For Review status.
2) Coordinator adds that approver in Scheduled For Approval status (Approval phase). But if actual approver for implementation phase approves it then the ad hoc approval goes for Close down phase.
So it makes no sense in adding that ad hoc approver.
Ad hoc approvers should be stored in field (like making a list of users/groups) till change moves to approval phase and once change reaches to approval phase, approval signature should be generated to all users in list with all must sign signature.