Making Default User while posting through BDC program through call transaction method

Hi Friends,

                Is it possible to set user as default user(Hardcoded) while posting F-04 transaction through BDC. Which means who ever runs program posting should happen only through the default user name.

            The program was developed through BDC Call transaction. Now Client was this Req with default user

          Please suggest me in achieving this ..

Thanks in Advance

Lokesh