I've come across this need - my DD classes always contain a Set Field_Related_FileField for every 'hard' relationship (well, they do in those where I know I'm using DD_relates).

Having come across Raveen's suggestion I think a simpler approach would be to add the following code to the Construct_Object method of your basic DD class definition so the DD_Relates are defined for all files. Of course this does mean you have to cancel the ones you don't want if only some of them are to apply. My experience suggests that it is usually only one or two relationships that you want to change with a pbUseDDRelates.
Code:
Class An_DataDictionary is a DataDictionary
    Procedure Construct_Object
        Forward Send Construct_Object
        ...
        ...
       Get Main_File to iMain
       Get Field_Count to iFields
       For iColumn from 0 to iFields
          Get_Attribute DF_FIELD_RELATED_FILE of iMain iColumn to iRelFile
         If (iRelFile) Begin
            Get_Attribute DF_FIELD_RELATED_FIELD of iMain iColumn to iRelField    
            Set Field_Related_FileField iColumn to iRelFile iRelField
            // Showln "Field_Related_FileField " iMain "," iColumn " -> " iRelFile "," iRelField
         End
       Loop
       ...
       ...
   End_Procedure     // Construct_Object
   ...
   ...
End_Class    // An_Datadictionay