Paul,
Hope you are using IS6.x version.
Select the UserDefined-1234567(as a custom ExternalId in the parterprofile)and when you are routing the flatfile to TN using wm.tn.doc.ff:routeFlatFile (input Stream,TN_params).
so before calling this service add a map step and build a record(TN_params this has childs set the values TN_parms/SenderID(userdefined,TN_parms/ReceiverID(userdefined),TN_parms/DocumentID,TN_parms/DoctypeName) and doing this TN will recognize ur sender/receiver/documenttype(FlatFile in TN) these names are casesensitive
Please test this case.
HTH,
RMG.
#B2B-Integration#webMethods#Integration-Server-and-ESB