your requirement is not clear,
Once you enter ME29N, hit on Disply/Change button user will able to see PO created by on Infobar,
Please check,
If requirement is to display created by field in mass release ie in ME28, then you can add it BADI/Implicit Enhancement with logic and help of abaper,
You can refer following blogs for it:
Add the field User Name in various Purchasing report's output ALV list