My understanding is that the workflow item would need to go to Manager's POWL inbox for the manager to approve. In addition, the Manager would then need access to GRC to approve the request. This would require an SAP user account with necessary authorisations to approve the request.
Parameter 5021 will allow you to choose a Manager who does not have a GRC Account and prevent request being lodged.
If you are using the function module GRAC_MSMP_MANAGER_AGENT there is a check which verified is the Agent is a valid user master.
I'm not too sure how you can avoid SU01 master for an approver without granting them the account.