How to make fields mandatory in ME51 for specific users

Purchase Requisitions are entered by each department at my client using Transaction ME51. The External Service Number and Vendor Material Number are not required to be entered for most of the users. However, they are mandatory for specific users (4 in total).

Is this possible to be done using User Parameters in SU3? I am not familiar setting User Parameters. Can somebody explain? Any other sugestion?

Thanks in advance,

Regards,

Marco