A Word Custom Document Property can be set in the merged document using the command SetCustomDocumentProperty.

An example would be:

<<SetCustomDocumentProperty(DocumentId,Contract <ContractNumber>)>>

This would assume that the data element ContractNumber has been captured in the data set for the document.  If in this example, the ContractNumber data element contains the value “ABC001”, then the above command would define a Word Custom Document Property (in the merged document) called “DocumentId” with the value “Contract ABC001”.