Hi Dharmin,
As you said Currently PO number is coming in your subject line. Please check the variable get populated to display PO number ( EKKO-EBELN ). To display the creator SAP ID you can use EKKO-ERNAM. I am not sure but if there is some way to put some logic to populate the variables used in subject line (In T-Code NACE) then based on ERNAM you can find the user name.
Also please let me know if this will not work then I can suggest one more option that will be complete custom solution.
Cheers,
Prakash