There is an option to set an approver for a supplier in its record where approval is done in Financials, and this can be done as follows:
In Creditors, find the supplier that you wish to assign an approver to.
Edit the record and select the Invoice Register Approvers tab.
Use the lookup in the Approver boxes to select the user.
Save the changes.
If approval is done in FinOps then the above option will not have any effect as approvers are assigned using FinOps processes.