How to implement interface type 'IFFIND' for object types

[Kouw, FA - SPLTX]

Hello,
 
We are running on 4.6C.
 
Tx 'SWI6 - Workflows for object' works well for objects of type
'BUS1001006 - Standard material'. When using this tx to find workflow
instances for object types 'BUS2081 - Incoming Invoice' and 'BUS2012 -
Purchase order' I get the following message: 'Selected object type must
support interface type 'IFFIND'
 
Interface type IFFIND has been implemented in standard SAP object type
BUS1001006, but not in BUS2081 and BUS2012.
 
I have 2 questions, maybe someone can help me out:
 
   * why has this interface not been implemented by SAP for these
     'common' object types (I suspect many people would like to use SWI6
     to see if, and if yes which, workflows have been started for an
     object) ?
   * is it possible, and if yes how, to implement this interface for
     object types BUS2081 and BUS2012 ?
 
Regards,
 
Fred Kouw
 
Ps. I found out that for all mentioned object types it is possible to go
to the material/invoice/PO and select 'Services for object - Workflow -
Workflow overview' to view the workflows started for the object.
 
 
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