Try something like this:
Select 'PO' as Doc,A.Docnum,A.DocDate,B.User_Code as UserCode,B.U_Name as Name
FROM OPOR A
Inner Join OUSR B on A.Usersign=B.Internal_K
UNION ALL
Select 'AP Invoice' as Doc,A.Docnum,A.DocDate,B.User_Code as UserCode,B.U_Name as Name
FROM OPCH A
Inner Join OUSR B on A.Usersign=B.Internal_K
UNION ALL
Select 'Outgoing Payment' as Doc,A.Docnum,A.DocDate,B.User_Code as UserCode,B.U_Name as Name
FROM OVPM A
Inner Join OUSR B on A.Usersign=B.Internal_K
Just add more document by making use of this sample query to begin with.
Regards,
JP